I'm running a 1996 LS engine with an OBD1 p75 ecu, would there be any difference performance wise if I swap in a OBD2 p75 ecu?

 

Not really. You're talking about a base model engine that wasn't designed for power in the first place. The tuning on the ECU certainly wasn't developed to push the engine in any way, shape, or form. If you want to gain what little you'll be able to with a basically stock setup, you may as well stick with OBD1 and get it tuned.