Hi. I just wanted to say that USB2 will not be a bottleneck in any normal recording setup.
It's more about the drivers.
I use my RME Ucx over USB2, and can easily run sub 5ms RTL.
All the best.

I missed the Punch In part of the situation, which would make manual anything impossible. The only workaround that comes to mind on that is to direct monitor the input (guitar) along with the DAW output and record to a new track. The "mute" could be done prior to the recording by splitting the clip and shift-dragging that to another new track (so you would be working with three tracks in that case). Not elegant, but would give you the ability to 1) "mute" at the punch point by removing the overdub region, and 2) allow you to get the punch timing correctly after the fact (would require editing the new take back into the target slot, but would alleviate timing errors between the DAW and Direct Monitoring should they occur).

Why is everybody making this so complicated for the OP. 
 
I think this is a plain case of the OP thought they needed input echo on to hear anything at all. 
 
All modern audio interfaces have direct monitoring capabilities. This is Zero latency because the signal goes directly to the headphones or monitors 100% analog hard wired. No A/D involved. This is mixed with the playback from the DAW. The playback from your DAW if using ASIO mode will be adjusted "ahead" of time to compensate for the latency the new signal will suffer going in. Result is an overdub in sync with the original tracks. 
 
I should not have to say this here as I would assume everyone has heard it a million times. 
Round trip latency is a sum of the total of everything in the signal path from the input to output, each component depending on quality adds it's little bit ( pun intended) 
A/D converters / The USB or Firewire system /The Computer system / Audio driver/ USB or firewire system/ D/A converters.  Most of these add very little latency with the Audio driver being the worst offender. Generally interfaces in the lower price points have hidden buffers that make supper low latency impossible. Most perform at around 7 + ms RTL at reasonable buffer settings. 
 
As the OP said, they have 20 MS RTL which seems what you would expect. Note that RTL will not change on different computers with different specs. This is another myth.  The only thing gained will be that a faster computer can handle lower buffer settings therefore you will have lower RTL without dropouts. 
 
The Soundcraft will probably still have the same RTL and this is nothing to do with USB vs Firewire.  It's going to use typical drivers.  Very few audio interfaces perform at the quality of those treasured few like RME. 
If you do not use in the box effects and guitar sims RTL is not worth thinking about. You will never notice it UNLESS you engage input echo. 
 
The Soundcraft is a great solution to having a multi channel interface and monitoring solution. I myself was looking at that board and it is a good choice. There are a few Sonar users here who bought one  and say it's stable and works as described. 

^^ Thanks.  The way I look at direct monitoring at the interface...it doesn't change when the playback hits my ear, nor when I move and play in sync with it - it only changes when I hear what I'm adding - my input signal.  That's it.  It's a change in where you hear your own input signal only - nothing else changes. 
 
Instead of hearing myself out of the DAW and back to the interface, I simply hear myself right at the interface.  
 
The DAW doesn't even need to know I'm doing this to adjust for latency.  
 
 
 

Just wanted to say I tried this last night and it worked perfectly.  Input echo was exactly what I was looking for.  Totally cool.  I was able to turn it on and hear the latency effect, and then turn it off and just hear myself right at the mixer.  Nice and simple for double checking your sound before you begin tracking, and then you can playback what you just recorded without turning anything on and off.  
 
I know it's a simple thing, but I'm easy.  

Right on. 
The cool thing about Sonar is you can learn something new just about everyday. There is always more than one way to do the same task too. We adapt to the ones that work the best for us and myself it is always the simplest method that works for me.