Apart from dropping in 850 motor there is not much you can do. I too have a 550. Mine has 26.5 rockers, QSC clutch, HMF pipe and HMF Optimizer, K&N filter and I cut a big as hole in my airbox. Other than the clutch kit, none of the other things gave me noticeable power increases, complete waste of money.
You know that sayin, "there is no replacement for displacement".
x2 on above. Nothing much out there to hop up an xp 550 or 850 for that matter that really adds power. With some cams and a bunch of motor work you can get power out of the 850, but you won't like the fuel bill to keep it alive. 116 is expensive!
Go with the 850 swap. My 550 motor is outrageously built, honestly nothing left to do to it (even custom internals) and I race it against 800s and 850s. Holeshot I was 3rd out of 9 but the 24v system helped a ton lol soon as we got into open fields, I might as well have put mine in park. I'll be trying to find a 850 later on this season do swap power plants.
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