Im Putting a 40 series flowmaster on my V6 3.8L. Just wondering if I should go with 2" 1/2 pipes from the cat back or 3". Which would give better performance?

3" give better performance, but are louder and tend to wear out quicker. Personally I would go 2-1/2" unless you were going dual exhaust, the 3" is fine because it shares the load.

Not dual exhaust. Single muffler but dual outs, one behind each back tire. They call it a camaro style or somethin. After consulting with a few people I'm going with 2 1/4" pipes 40 series flowmaster. I should get the best performance out of that, Its a stock V6. I'll let ya know how it comes out.

3" would be too big for a stock motor.

2.5" would be perfect