2015 Kx250f low power / weak

Okey, i started to ride on 250/2strokes back in the days and then been on 450/4stroke when they took over, of course tried 250/4strokes when switched bikes with friends in the past and remembered i thought that the -14 kx250f had a strong engine for a 250 and i was very comfortable on it so i bought a 2015 kx250f just cause i wanted to ride it for a season to see of i like it best on a 250 or 450🤷🏼‍♂️ 

tried it out last weekend and it wasnt like i remembered it when i tried it 10years ago, how i remembered it was strong (for a 250) all over the powerband, revved high and was very strong on top. My 2015 just felt weak, didnt feel it wanted to rev very high and more signed off instead of a hit on top? With this said, it could just be in my mind but i would like some tips and ideas of what could cause this?

The sidepanels of the airbox are broken of in the middle so there is free flow into the airfilter, gonna repair them til Next time but figured it was ok since i rode on snow/ice at the track just to try the bike once. Could it be a problem that it gets too much air that the afr runs lean?

the exhaust is really loud but from my memory both the 250 and 450 kawis had a very loud exhaust? I’ve ordered new muffler packing and Will repack it til next ride, how much power would get lost with blown muffler packing generally speaking?

Could the clutch be slipping and cause the low power feel? I’ve never been good at noticing this on dirtbikes so i’ve ordered a new set of plates, discs and springs that will be replaced for the next ride.

i’ve set the valve clearance when i first bought the bike, double checked it today and it was fine, double checked the timing and the marks line up perfectly, also checked the cam gears to the camlobes, they are were they should be and have not spun.

Rode on a new set of budget spike tires due to the icy ground, wasnt the best grip but not that bad either so dont think this caused it but Next time it will be on regular summertires.

i had a new airfilter, new oil & filter, new gasoline, plugged in the white coupler from an old 2012 kx450f i had, i had cut out the fire screen behind the airfilter

i have not checked the compression since its so easy to start, had it in a friends bus over the night (winter -15c) and still starts on second kick. Should i still check the compression? Could this be a problem even though it starts so easy?

what have i missed? Any ideas? I’ve really been looking forward to try riding a 250 for a whole season and this was just really frustrating😅 and i dont have any 250 friends to swap bikes with no more, all ride enduro or bigger bikes.