I have a fuel pressure regulator for my stock 2.8, now what would be a good PSI for getting the most performance out of the engine?

 

I would say set it about 43-45 psi running.

 

You set it to where it runs best. But you should not set it too high or you'll be replacing your fuel pump. But, in all honesty, simply increasing fuel pressure doesn't mean an increase in performance. You might feel a little increased throttle response at part throttle, but that doesn't mean it's performing better overall.

The best way to utilize an AFPR is in conjunction with an eprom. I do believe in higher fuel pressure, but if the engine is bone stock, you'll most likely be decreasing performance due to running too rich. GM (by design) tends to make these cars run richer than normal due to warranty concerns.