From what I have heard, the shiftpoints are controlled by the ECU. Even from personal experience, I have seen this (won't shift to OD till a certain speed, converter doesn't lock up till 37 MPH, and so on). I don't know if there is a way to make the ECU provide a 'crisper' shift, but I definitely see a need for it in MY xB. Maybe there are some internal 'tricks' that can be done to provide this, but I don't know if the ECU will allow it.
We're working on it at the shop this week since I work at a trans shop. I've already messed with the accumulator springs, droppping the valve body and drilling next, and as soon as it's tuned I'll have pics and part numbers. Have you seen my trans cooler, it helps get me over the Baker grade at 100mph and the temp is only 200F with AMSOIL. The stock T-IV boiled over and left me in limp mode the time before.