Will a midi mate control guitar rig 4 pro

I'm still new to sonar but in cubase you can map note information from an external device into a command for most anything.

But it is positional in cubase, not sure about sonar.

So if you set it up to control things in Guitar Rig and it's the 4 plugin on the bus then you add another plug in and make GR the 5th position you have to reconfigure it.

You need to find out what messages the pedal is capable of sending then find out what messages the plug responds to etc.

A box like is made by midi solutions can convert one midi message into another one. Like my yamaha pedal sends program changes or CC's but the MSbox converts that to notes which cubase converts to commands.

I use it to change cursor tools.

  If your computer is fast enough you should be able to get the latency down low enough to be unnoticeable without any dropouts. Latency shouldn't be as noticeable in a live setting as compared to recording too so that gives a little more wiggle room and to keep CPU usage down and you can leave the "Hi" button off because the difference won't be as apparent. As far as the controller... MIDI is MIDI. If the parameter in GR you want can be assigned you should be able to control it with whatever you want. However the GR controller is probably the easiest if you can afford it.  

  That said though, TH2 is probably better live for multiple reasons but the main one being it is less of a resource hog than GR which in theory should allow you to get your latency values lower. Obviously it is fully MIDI assignable too and although I've never assigned either program to a controller the TH2 way looks a lot easier. TH2, IMO, also sounds a lot better and will likely react more like a real amp on stage.  
There is a video on youtube of a guy showing how the TH2 MIDI assignment stuff works. Just search "TH2 MIDI" and it should come up.

Cheers.