I believe the only restriction is based on an imposed 92 dB sound limit. At full song, it's not hard to imagine most aftermarket exhausts exceeding this limit. So people out to enjoy their trackday resort to all sorts of rigging, from additional glass packing to installing adjustable exhausts, just to meet that requirement.
__________________ Most people would... "Um, I'm not most people." Okay, so we've traded bikes for a while [Damn, your bike's a POS! ], now give me mine back!! Ahh, what a sweet ride. Setup link(s): K&N, Supertrapp Aluminum Racing Series tunable slip-on exhaust, jetted, stock gearing & ignition. ZX-9R MPT max range ~266 - 275 miles & MPG tracker at Fuelly.com
], now give me mine back!! Ahh, what a sweet ride. 
