These 6spd transmissions can be very finicky. We have had quite a few of them on the dyno, and now that we own a 2010 Denali, do a lot of playing around with the tuning on it. Like it was stated, the TCM tuning will make a big difference in the driveability of the vehicle.
Most customers with the newer vehicles that have 6spds are going for the AutoCal option, as they can datalog and also have the ability to flash the custom tunes to their vehicle. It is a true custom tuning handheld- no downtime.