I personally like the idea of the subframe brace, especially given the problems that some have reported with the IRS wheel hop. As far as high speed stability I'm not sure if it will be explicitly helpful.. The G Trac brace claims to improve cornering stability by strengthening the control arm mounting points but again, I don't see this lending itself towards high speed stability.
I guess my biggest question is does the car feel unstable at 100+ mph? If so, what type symptoms are you concerned with correcting?