There was an Engine Masters dyno test on this topic, and many years ago a magazine test in HotRod or CarCraft where they did a similar test.
I think the magazine used a 3rd-gen Z28, probably with a bit less power output.
Gut feeling is if the OP's engine is done @ 4500, 2.5" should be OK - muffler's likely as important in that scenario.
If it spends much time at 4,500 - not likely on the street - then 3.0" would likely help.
3.0" will also be louder...