I went straight to GTR cars when I got S2, so here's my little feedback on those:
For XRR the most important thing is to get turbo pressure up before launch or otherwise you stall for sure, but downside of FR layout is that you'll be lacking rearwheel grip compared to FZR.
I usually do burnout while lights are counting down, atleast I don't have to over-rev engine to get boost up and I get warm rear tires for getting power down faster in T1 exit.
FZR on the otherhand is just amazing off the line due to weight distribution, just floor it and go. I've set 1st gear ratio so that there's enough wheelspin to get moving and get revs high enough before grip returns.
As for FXR, can't say much because I don't drive it but it seems to be pretty much equal to XRR and you can stall easily with that 4WD.