Can anyone please tell me the difference in power between a 13 and 15 tooth front sprocket? If I change stock to a 13 what will I gain or lose?  

if you go with a 15 tooth sprocket from a 13 tooth, you'll gain maybe 3-4 mph on top end.
the difference is not very noticeable. however it does help a little with running with bigger machines and not having to run in say 2or 3rd gear all of the time. so it makes it a little better for trail riding.