Here's the problem, Si tranny has too close of gears for an LS engine, if you don't want to go faster than about 110 mph at the most, then an Si tranny will do, most prefer a different route, I was very happy w/ my LS engine and GSR tranny. it's a very good combo, if you plan on going an all-motor route, I'd recommend a GSR tranny for an LS engine. The only problem I ran into was that when I sprayed, I wished I would have kept the LS tranny, In a HB w/ an LS and GSR tranny I had a respectable car, but when I sprayed it was very fast and I had to keep shifting. I feel that if I would have kept my LS tranny I would have been faster on the bottle. So...
All-motor LS= GSR tranny hands down, not too short but not too long legged
N2O LS= LS tranny
The only way I'd recommend an Si tranny is if you had a REAL built LS/VTEC, and by built I mean it, the Si tranny's gearing os for high rpm action and it will destory and engine because of how tempting it will be to stand on he gas and hit 8k or 8.5k all the time.