there is a certain amount of throttle that can be depressed to attain a 6-5 downshift, but it also depends on load and ambient conditions. you have to remember that these cars are tuned for fuel effieciency, not balls out performance. they have to abide by CAFE standards. there are ways to tune the transmission shift adapt timing sequences, but you sacrifice fuel economy, and you also have to remember that computers are stupid, they can only read 1's and 0's. they abide by tuning specifications. if you change the parameters, you effectively override the factory torque limiting system and now make the very expensive 6t45 transmission sucseptible to damage. the factory tuning is more than adequate, you just have to push the throttle a little harder and faster to attain what you want. in the old days where you had a TV cable attached to the transmission pressure governor from the throttle bracket, it was easy to manually make the transmission shift in accordance with throttle input. today, its all for emissions. if you want go, you have to mash. the other thing you have to remember, is that it might be going into 5th gear, but you cant notice the shift due to load condtions, and the reason it feels like its staying in 6th gear is because the car is also utilizing cam timing. you have 4 cams that can vary 25 degrees + or - static TDC to attain whatever driving and loading conditions the driver inputs to the ECM. when it feels like its staying in sixth, its downshifting to 5th, but you cant notice it because it has now retarded the exhaust cams and advanced the intake cams to provide for greater valve overlap in an attempt to retain fuel economy. when you push harder on the throttle, cam phasing then will change to a performance standard and advance the exhaust cams and retard the intake cams to reduce valve overlap and increase cylinder pressure to increase performance and now fuel economy goes out the window. well, i hope that helps you all a little bit. and one more thing, if you decide to retune the ecm in these cars, and you override the torque limit system, and you break the transmission, and you take it to the dealer, and they check for failure records, and they see the tuning calibration has been changed..........your warranty goes out the window. gm will say, go pound sand, and then put a warranty block on your 100k warranty that you paid for when you bought the car. that warranty block shows up on gm's VIS website and no dealer in the country will warranty anything on your car at that point. so dont do that unless youre rich and dont care about warranty.