I know that almost everyone with a 96+ GSR that goes turbo, replaces their OBD2 ECU back to OBD1. Why....and for how much money do you do this?

 

It's not limited to FI setups either. I switched from OBD II to OBD I primarily to use Hondata, whose use on an OBD II ECU is not very practical. Cost was approximately $325, which includes a $150 adaptor harness and $175 for a P28 ecu.