I had a friend at work that purchased a GMAF for his 2001. After 4 or 5 months of trying to get it tuned he put his OEM back on.
According to a local LS1 tuner (
http://www.forcefedperformance.com/) a lot of people make the mistake of trying to feed the LS1 more and more air like a ford SC motor. These LS1’s have plenty of air you need to attack it with tuning and timing.
He took this same car with the OEM MAF, a BPP vortex intake,x-pipe, Corsica Indy exhaust. And tuned it to 345 HP at the wheels. The best Performance Chevrolet in Sac could do was 305 or 310. I know the dealer is limited as to what they can do, but this was a significant increase with just tuning.
You might want to shoot off an e-mail to Keith (I think that was his name). He may be able to explain it more correctly. This info was from a few conversations I had with him back in Feb or March.
John