I found nothing on the topic... instead to going down a tooth in the front on my SF, would I see much of an improvement just using a stock 42 tooth rear sprocket from the klx250s??? I know I wouldn't need to change the chain going down a tooth, but how about going up 3 in the back?

 

I don't know if it will help you, but I did just the opposite.

I ran a 39 tooth rear from an SF for a while on my 2006. I had to remove one link from the chain.

Ron