Yup, it's really just a quiet rumble when pulling away from a stop ... that is, of course, until you NAIL the throttle... that's when you'll need the earplugs 
The 54mm would work according to the dealer...UndergroundLare said:What system comes with the 999s anyway? All the websites say that it includes a separate Termignoni Race Exhaust. But they don't specify whether its just a slip on, half system, asymetrical, full, titanium, or stainless, 50mm, 54mm, 57mm.
Edit: Well i did some research and it looks like it comes with a Muffler, air filters, ecu, and a header for the horizontal cylinder. But i'm still not sure what diameter it is and whether that system would work on a base 999?