In fact the gains are almost non existent. There was a 3071 car that did back to back dynos of 58mm to Robs (62?) Or it could have been stock TBs to one or the other but even at the 500hp+ level they only gained ~5whp which at that power level isn't even noticeable and the gains would be even less at lower power levels with smaller turbos. I believe the heads are the bottle neck not the TB's. So maybe with sufficient head work the TB's would help but IMHO until I see hard evidence I wouldn't waste the money on them.