I am thinking about putting a dyno jet kit and K&N fileter on my 300ex bored .80 over, is that the most bang for $100 and how will effect my 300 ex?

 

You don't just buy a "kit" and throw it in. The kit will come with general recommendations for what to run with different mods, but that is just a place to start.

Jetting is art not science!!! It is done by trying different jets and riding the quad and seeing what they do. Have it done by an experienced pro...if you aren't up to it!!!

I wouldn't just do a K&N without doing a muffler (and header on the 300!!!) as well.