Do it. I got plenty out of my apex downpipe on my 2G, I think it was 2.5" then opened up a ways back to 3". Testpipe or resonated testpipe ftw, you'll be pick up some good gains with it.
What other mods do you have? Boost gauge and MBC would round you off with a nice, responsive setup
It's all downhill once you do that stuff though...the mod bug will have you!