Since you installed a new trans, and a performance one at that, perhaps the memory in the computer that has learned and adapted to a worn out transmission could be causing the "to hard" shifts.
I found this out to be true with my car, but just the opposite happend to me. I was
tuning some parameter in my car "SHIFT TIMES", with my JET DST, and accidently pulled the main fuse to the PCM before the reprogram, and when I went for a drive, the shifts were real soft at very light throttle, but ok on mid and full throttle.
The trasn has 150,000 mile on it, and was shifting ok, but I wanted to just frim up a little bit was all, but when I reset the PCM by accidently removing the fuse, all the miles of learning was lost, so all I did was add a few PSI to base shift pressures on the low end of the scale, and everything was better again.
