Hi all,
 
After months of dealing with this, Waves told me today that Sonar is not officially supported with the DiGiGrid IOS, and they will not be prioritizing any bug fixes for it.  Therefore, anyone thinking about purchasing one should realize that you may have problems offloading your plugins to the SoundGrid server.
 
Waves actually suggested moving to another DAW.  I've been a long-time Cakewalk and Waves user, so the thought of moving to something else doesn't really excite me, especially since I paid into lifetime updates for Platinum.  I don't think my problem is due to a lack of resources:

ASUS X99-A
Intel Core i7-5960X Haswell-E 8-Core 3.0GHz
32 GB of RAM
Samsung MZHPU512HCGL SSD System Drive
WD Velociraptors for project audio and samples
 
I've submitted support requests to Cakewalk (including a bug fix request, in case Platinum had compatibility issues with it), but Cakewalk have not responded to or addressed anything.  I still suspect the problem is with a change Waves made in their drivers starting with v9r11.  I'm otherwise happy with the IOS interface, so (unfortunately) I will most likely be testing another DAW.  I'm not sure of any audio interfaces that are comparable to the IOS, and the Clarett 8PreX still does not appear to have Windows drivers.
 
Best,
Stephen Salazar
 

I found out the issue isn't with Sonar, as I was able to reproduce the problem with Cubase 8.5.  After much troubleshooting, it appears loading Butch Vig Vocals or Manny Marroquin Reverb in a StudioRack insert that is running from the SoundGrid server affects the DSP drastically.  I'll post more here once I'm closer to the definitive answer.  I still suspect a bug, unless it's a compatibility issue with my NIC, but disabling both of those plugins, saving the project, and restarting Sonar improves performance with the IOS dramatically.

Best,
Stephen Salazar

Waves got back to me and said they were able to reproduce the problem.  Sonar is still not officially supported, but it works as expected.  The best workaround is to use Waves plugins that are CPU-intensive and offload them to your computer's CPU (it seems a lot of the artist bundles fall under that category).  Adding lower intensity plugins in StudioRacks and offloading them to the SoundGrid Server is a better way to approach using the DiGiGrid IOS.  I've requested for Waves to update the list of plugin benchmarks on their site to get a better idea of how the higher intensity plugins tax resources.
 
I hope this information helps.
 
Best,
Stephen Salazar

Too bad that Waves is not supporting their Soundgrid platform very well.
Did they say when they would fix this ?
Since it also affects Cubase, hopefully the fix will help Sonar as well.

I use a LOT of Waves in Sonar so I was really hoping I could leverage Soundgrid.
Not going to go there until they fix things.

Recent updates were supposed to address overall performance issues, but I think it really comes down to how you use some of the artist plugins (such as those from Manny Marroquin, Butch Vig, and Chris Lord-Alge).  There wasn't a lot of data on the Waves site at the time showing how well their servers handle some of these plugins.   I suspect they are system-intensive and are probably not going to work well with a large project with lots of plugins loaded on a server like the IOS.  If you're loading the lower-intensity plugins (based on Waves' comparison charts), server performance should be dramatically better.  I'm just learning to balance plugins I load on the server vs. plugins I load from my computer's CPU, and I'm finding that's a pretty good compromise.
 
By the way, Cubase is officially supported by Waves for the IOS interface / server.  Sonar is not.  However, it still works.  The only major thing that doesn't work has more to do with the fact that Sonar doesn't support Mackie HUI, which would be super helpful, but it's definitely not required.
 
I kind of wish the server was a bit more stout, as the processor could have been faster, and more RAM would have been a plus.  I will say that the interface sounds great, and its software gives you a ton of control over how you route audio.  It's pretty impressive.
 
Best,
Stephen Salazar