I have created a project and loaded it up with a ton of VSTs and VSTis, mostly Native Instruments. Wanted to see if there are any stability/performance issues before purchase.
 
I started at the lowest buffer settings (48 samples, 4.3ms round trip) and as the load increased gradually and introduced distortion, kept increasing buffer up to (512, 25.4ms)  SONAR surpasses my expectations in plug in performance.  My computer is an old PhenomII x4 @3.7 w/ RME UFX over usb -- I was expecting less instances.
 
For VSTis I have a couple instances of Kontakt 5, EZDrummer2, Maschine, and Guitar Rig 5.  There are about 50 other random plugins (mostly compressors and about ten light verbs) and I havent had any crackles at 512/25ms.
 
Surprisingly I was able to load it up with Kontakt, EZDrummer2 and about 15 plugs at 48samples/4.3ms, before having to move to 64samples/5.1ms.   I dont know if this is real world performance because the project only has a handful of audio and midi clips.
 
The strange thing I have noticed is that the CPU performance meter (4 cores) only gets to about 50% before I start to hear crackles.  Is there a setting in Sonar or system setting that would  make this more accurate?  Perhaps it is assuming 2 threads per core like Intel.
 

Probably a good ideal to put your system spec in your signature, including operating system etc (see my sig).

If your machine is Intel consider turning off Speedstep at the BIOS level (there is an AMD equivalent). Optimize your Windows power settings as well by adjusting min/max core settings (this is what it's called in Windows 7 anyway, windows 8 it is different). You may need a reg hack for this (google for details).... you don't need to adjust the windows settings though if you turn off speedstep at  BIOS level.
 
Otherwise it could be any plugin slowing you down (you can rule it out by removing instances), or bad drivers/firmware or you need to do some latency optimization. Or maybe you need to optimize windows (chkdsk/defrag). There is a wealth of information in these forums about how to handle this. The question pops up every couple of weeks in this forum.
 
Cheers...

The machine is exceeding my expectations, I'm just wondering why the meters are only showing about half before it starts to crackle. 

Yes I read your post. Please check #1 esp speedstep. I'm asking for system specs for my benefit so I can deal out more specific advice rather than having to waste time covering all bases. Time is apparently money or something :) You have the generic advice anyway posted in #1. Thanks and good luck..

Oh and I forgot there are a couple of options in preferences including using 64bit precision engine, and there is another multicore engine setting hidden somewhere (google for specifics). That and speedstep you need to check.

Teal
The machine is exceeding my expectations, I'm just wondering why the meters are only showing about half before it starts to crackle. 

The CPU meters in Sonar show how quickly the buffers empty. Not the actual CPU usage. It is a more accurate way to predict performance within the DAW. However there is a switch in the audio ini file I believe that can improve core balancing. 
 
Below is a copied sectioned from a post from this thread. http://forum.cakewalk.com/Multicore-use-dedicate-first-core-for-only-OSDAW-m3000964.aspx  Post no. 5. It still applies I believe.
 
"9- Sonar Tweaks:
* Improved Multi-Core load balancing [From Noel's SONAR 8.3 Log]

Since all systems are not made equal we decided to make this new scheduling mode configurable.
From the help file, the new aud.ini option to control this is:

ThreadSchedulingModel=<0 - 2>
Type Integer
Default 1

This variable goes in the [Wave] section and controls the interaction of the main audio thread and worker threads on multiprocessor systems when the Use Multiprocessing Engine option is enabled. Depending on the system, a particular model may result in less glitching and better overall performance. The values are as follows:

............0 = Same as previous versions of SONAR.
............1 = (default) Better thread balance. Model is more efficient and can provide cycles for other tasks.
............2 = Additional worker thread is created. This may result in improvement with Quad processor systems or higher. Not recommended for Dual processor systems."

It's cool to check out these types of things but AFAIK the performance module is meant to be more of a general idea of how much resources are being absorbed while you work so you can spot problems before they occur. It is not meant to replace a proper resource monitor. You should always allow plenty of overhead when dealing with audio (or video) anyway. I have never had any of my RAM or CPU meters even get close to 50% even in huge projects with tons of plugins in X3. The only issue I have had is with disk read/write speed and that is easily resolved with increasing the read/write buffers.
 
I understand kicking the tyres of a potential purchase but come on... now the Perf Module is a matter of debate?
 
You can get the base version of X3 for peanuts (especially if you wait for a month or so) and with your plethora of third party plugs you won't have to worry about those "dated" sounding plugs. You'll also get all those neato keen sample projects you want (because apparently even as a busy pro you don't have your own material to load into it).
 
I am probably the biggest fan of the perf module on this site but good gawd... what the heck do you want? I full real time diagnostic of your system WITHIN Sonar? That in itself would likely cause problems for your recordings.
 
Ridiculous.

If you're getting dropouts with the CPU meter at 50% that suggests something in the system may be hogging a resource, and not necessarily the CPU.
 
Network and video adapters are frequent culprits, since they assume they're at the top of the food chain and tend to monopolize resources at the expense of audio. Try disabling your network and see how much difference that makes. In the case of wireless network adapters it can be night and day. The smoking gun is high DPC latency, which doesn't show up in CPU meters but can be measured with a tool such as LatencyMon. 
 
High-end video adapters designed for gaming can also be a problem for DAWs. The presumption is that video refreshes are what you care about most, an assumption that makes sense for 99% of computer users but not for us audio geeks. They also have fans on them that are not your friend when you need quiet. So if you've got a gaming video card, sell it to the kid down the street who plays Halo and buy yourself a basic card. 
 
Also, your test involves a lot of sampled instruments, which means your disk drive(s) is a major factor. You could be having drive contention and disk thrashing, or even paging occurring. This is likely if all the Kontakt instruments,EZDrummer samples and your project audio are all being read from the same physical disk. The fastest CPU on earth won't help you if your disk I/O can't keep up.
 
It's normal to see all four cores not being utilized. SONAR does an admirable job of load balancing, but there is only so much it can do in that regard. Remember that it's spending 95% of its time executing code that's not even part of SONAR.
 
BTW, on my aged, underpowered 2-core machine I routinely hit 90% CPU here before experiencing dropouts/crackles/distortion.