My kit came with severely restrictive 1 7/8" ID stock baffles and I had the car dyno'd before and after changing to 2.5" ID baffles, louvres facing the rear. The gain on that change alone was about 45 HP but then I'd been choking my BB with the mufflers that came with it. I think I'd pull up a few more ponies with 2.75 ID pipes but I never race anyhow...and I've already got more under my foot than I know what to do with
The sound with the restrictive pipes was brash and irritatingly loud, the sound with the larger baffles is more of a rumbly growl, a far more pleasant tone which invites many an inquiry as to "what've you got under there?"!
I found and used an exhaust pipe diameter calculator that took into account the Engine CID etc. but I can't find it now, just Googled it luckily.
Regards!
Dirk