I have an 01 250ex and i was wondering if getting a ventilated air box lid would add much power. I was also wondering if i would need to rejet.

 

It will add minimal power without also piping it and yes, you will have to rejet. I'd say go up one on the main.

 

I have a white brothers e-series pipe, and a white brothers powerfilter. Does that change anything about having minimal power and going up one jet in the main?

 

The little 250 can only manage so much air. A couple of 1" UNI vents along with an FMF Megamax II muffler is all I could do. Any more than that and it wouldn't jet right.

Or, look at the intake snorkle. The hole in it is very small. You could open it up some, and do the same thing.

 

I agree with what recon said. The 250 is hard to jet if you give it too much air.
My wife's 250 have 4 filtered vents and a FMF Megamax II and I bumped it up I jet size.
I will rev quicker and hit the rev limiter fairly quick after a rejet and pipe/filter