First, you're not going to gain much performance with the 3 speed, you just lose the OD. The ratios aren't much different, nor is the HP drain.
Second, the computer is going to miss the inputs from the trans. The speed sensor can be put on the 3 speed, but it won't be shifting when the 'puter tells it to.
I'd just change rear end gears to get the RPM's up. With gas prices these days, that OD is nice!