i've got a 57 full system on the 999S. its a bit louder than stock but not offensive and it feels like its got a bit more mid range but I haven't had my bike dyno'ed so can't say for certain. i'd have preferred a 54 system, most of the tuning info i've seen says that the 57 is really for a more highly tuned motor and has little benefit in road use. Some tuners even say the 54 is better!! the 57 is too big and catches the fairing so you need to tweak brackets, it also rubs the swingarm and i had to space my pegs out on the right handside, not what i want to do as even with them set high up on the mounting points i still touch them down occasionally. it took me a lot of cursing and swearing to fit it, its a tight fit and you need to position it loosely and then start tightening bolts, even then you'll find you need to tweak it.
RRP on the one i bought was almost 2400 inc ecu, i never fitted the new ecu ( on the advice of Ducati Glasgow) if you are buying S/H check whether ecu has been installed and coded to a key, if it has its useless in any other bike and a new ecu is about 700.