I'd buy a tuner if I could remap the gear change schedules. From what I've read the Supertuner one only allows torque management adjustments and shift timing on the stock trans programming, no gear selection changes.
hpc3: I'm not sure if this is exactly what you're looking for, but the superchips advanced transmission tuning allows you to make your own shift schedules. I'm not sure if you gain torque converter clutch lockup control or not, but many on the superchips forums use it and love it for towing, you may want to look into it. It's an additional fee(like $50) on top of the tuner itself, but everyone I know that bought it loves it. I have yet to buy it and don't know that I will as I rarely tow and the preprogrammed shift schedule in the tuner is fine by me.
EDIT: also, some members on this forum have commented on it, I believe user Drake has it, and maybe ORT, and others...?