Most people seem to get 3".. but I get the feeling this is incase you raise the boost a lot higher. I'd run 11psi MAX at the track.. but most likely 7-8psi. Will a 2.5" do without affecting performance too much?

 

3"

 

You could do fine with 2.5" IF you had the wastegate dumping in its own pipe, aka, not routing back into the main exhaust...

if not, go 3"