Hi there ,
my honest advice , heres a long story !
The 300 kit will give you a beter ride , it has much more midrange and pulls beter than your 250 ever will .
To make your 250 perform like a 300 - you will spend more than a 300 kit .
Get your self a exhaust gas temperature gauge to sort out your jetting .
I have raced hard with the cast pistons and the wiseco pistons and both have performed well .
Forget about comparing hp on the dyno and feel the difference when you ride the bike .
You are welcome to test ride my bike anytime to come here - no questions asked .

Ask plenty questions and we will get you sorted out .

cheers , Neal