Stillen's kit uses an UpRev canned tune with their "turn-key" kit. UpRev uses the MAF sensors to actually tune the car, just like a factory tune. Think about it, we buy these cars worldwide; different elevation, different gas, different variances.
When you're tuning the vehicle, the MAF sensor is compensating the air, so if you say at full throttle I want to keep it around 11.5 - 11.6, then the MAF will compensate the air/fuel to see as close to those measurements as possible. Why do you think when you go with GTM's turbo turn-key kit, you don't need a custom tune sent to you if you decide to add test pipes, a 3" exhaust or anything else (thanks to the blow-through style MAF setup)? A lot of it has to do with Timing too. If you don't have an aggressive timing, you can get away with extra mods without needing a new tune.
What I think the issues is... the pull-through MAF system. You're not getting an accurate measurement of the air that's reaching the engine because it's being changed when it blows through the charger.
I've done a lot of research on this and there are a couple members I'd like to personally thank that have given me great insight on their review of this kit.
I could be wrong though
