Vegas doesn't seem to have any issues with hanging on large projects, though to be sure my usage of it is more limited than some people. No audio limits of any kind to speak of in Vegas, since its original heritage is actually a DAW, not a NLE.
Only thing to note is that the included audio effects tend to have a design on speed, not quality. It loads DX and VST effects though so you can use any others you have that you like without issue.
Also it supports buth CUDA and OpenCL acceleration of video effects, but with cards like you have, it will probably be faster to leave everything on the CPU (you can tell it how to handle things). Also in the past I've had issues with OpenCL, effects crashing when using AMD cards. I've had an nVidia card at work for a couple years now so maybe that is fixed, but I had a lot of trouble with OpenCL accelerated effects, hence the switch to nVidia and CUDA.