Couple of things I can go by is someone else's research but research the same,
Dixon Cheng from Dms only supplies 2.5" exhausts to his n/a s2000 customers because 2.5" is already maximising the flow that the f20c can produce,
So what would make any other 1.6/8 or 2.0 any different?
Besides the extra cost in 3inch you've got clearance, noise, weight. It's just not beneficial when a 2.5 or 2.25 can do the job.i might be wrong but doesn't the B16b runs 2.25"? If that's enough for it to flow 180odd hp then 3" is overkill.
You may not lose power top end from going from stock to 3". But if you compare your average joe on here with bolt ons going from say ther skunk2 or buddy club or whatever to a 3" is waste of time. I'm no exhaust expert but I know for a fact 2.5" exhaust can flow enough to support over 300hp as I've done it. In fact I had a restrictive centre section when I did that on my Astra.