How much Power/accel will i gain if?

[07250ex]

If i have a full hmf sport series with a quiet a core a uni/k&n filter and 20 inch razr tires jetted correctly ... over stock

[tedwilley]

I have no way to quantify the gains, I don't know that anyone would spend money to dyno a 250ex before and after the mods.
But, you will feel the difference in the seat of your pants.
Stronger pull in the low end especially. Faster acceleration due to the lower overall gearing from reducing to 20" rears. Top end will drop off in fifth due to the smaller tires. Whether you will notice that is relative to how and where you ride.
Also, you will thankfully lose the lawn mower sound!
BTW, I put on 19" tires and they lowered my gearing too much. It accelerates great, but I lost the effective use of first and second gear. I now spend all my time in fourth and fifth, and on flat out runs, I want to shift to the missing sixth gear. But, I can also break the tires loose at any speed.
I'm going to 20" rears at the end of this season.
If the small top end loss is going to be a problem, get a rev box.
(AMR, RM Stator). The added 1000-1500 RPMs will more than make up for the reduction in tire size .