[Tom Goldman]

A lot depends on how much line pressure not how much engine horsepower .
If you are using a trans brake valve body and the line pressure is around 200# or more, I would definitely use at the very least the 28133HD return spring .
If you see any signs of what appears to be band slippage or overheating from shift overlap ,particularly on the middle of the band ,you might consider going to the 28821-10K ratio servo.
I have seen major improvements on band wear with that servo.
Significant enough that it can be seen on a data recording graph