a lot of 999 guys are saying that 54mm is the ideal diameter for the standard testastretta motor withough headwork. the 999S comes with hotter cams and larger valves. for that bike, the 57mm system seems to be the hot ticket.
i'm looking at the prices of these systems and [email protected] my pants because i'm going to have to come up with a new header/mufflers for my s4 when i stuff in the 998 motor.
with my s4's stock 916 mill, i'm running a custom setup derived off an ST4's header. the ST4's stock header is 45mm as opposed to the s4's stock 40mm. but there's no way in hell i'm running ST cans. they're f'ing HUGE so i made my own. 50mm tubing leading to muffler bodies off 748 donor cans cored with 52mm perf'd. good stuff!!! 45-50-52mm low mount full system with over 10 lbs weight saved compared to the stock anchors on the s4. can't wait to dyno it.