Yeah when it comes to the 550f, there are things you can do, but the gains will be minimal, and hardly worth it. The more you push out of fan cooled motors, the hotter they run, the less constant power they have.
I mean, there is a bunch you can do, but it seriously is not worth it.
Wait a sec....Ski-Doo never offered the 550f in the S-Chassis. Only the 503. The 550f came after the ZX chassis was released.
In any case, with either chassis, the best nag for your buck would be a higher seat, different handlebar setup(read risers and bars), and skis. Otherwise, do not spend anymore money on that fancooled motor.
Way back when we tried one of Cycle Works 503 mod engines. Welded cylinders, high compression, 40mm carbs, twin pipes. Tow rope was the order of the day. Ended up with mild port, slight comp. increase, stock pipe with a reducer sleeve in it and a homemade silencer. Lightweighted it like crazy. Kinda fun but a stock 500 Summit was way way more fun.