A few factors go into determining shift firmness including line pressure. Also shift times, torque management settings, input shaft torque, separator plate setup, valve body springs, etc. With just a tune and not actually opening the transmission up, you can raise line pressures a little (need to make sure you dont raise them too much and overwork the pump), decrease shift times, and remove some/all torque management. If you drop the pan and install a shift kit with servos, different springs, drill the separator plate, etc then you increase the pump/line pressure volume mechanically which firms the shifts up.
For those wondering if just adding a shift kit and leaving the tune alone is ok, it is BUT you'll get a lot better results with a tune as well. My 02 I6 has a shift kit, Vette servo, and my tune and the shifts are perfect IMO. Nice and firm at part throttle, but they dont snap your neck driving around town. When you punch it, then it shifts quickly and much firmer. I probably could make it shift faster, but honestly I have no reason to and Ive been very happy with how it shifts for the past few years so I havent changed it.
Now the SS is a different beast, a tune helps a ton but they definitely need a shift kit to help handle the power. Im probably just going to run my stock trans as long as I can, then get a fully built one.
