I understand there is a tool Lingenfelter makes in order to unlock the factory limit. I read it in GMHTP I think. I think that was ADM's 'V pulley, intake, tuning article.
If you go to a reputable shop, like one of our Sponsors, you'll be just fine. You get what you pay for, though. I definately wouldn't go anywhere that doesn't do a lot of tuning on your specific car.
05 Hummer H2 SUV on OEM Air Ride
Forged 11.4:1 408 LQ4 (K1 crank & rods; Mahle pistons)/CPP LS3 CNC heads/BTR Stage IV LS3 cam/home ported L92 intake manifold/ported 90mm TB/custom Volant CAI/mid-length headers/AFE dual 3" CB/Corvette Servo/el cheapo lift/Cognito UCAs/Sphon poly adjustable TA/Sphon adjustable rear LCAs/e'fan conversion/DirtyMax radiator/Moroso catchcan/HPTuners/HID conversion/LED exterior lighting/LED interior accents/Autometer Cobalts/AEM WBPioneer AVH-5600BHS/Pioneer GM-9601/2-Pioneer TS-W310D4s/Obcon Labyrinth dual-12 box/lots of other little stuff