Well the 6400 is the number one easiest to hot rod saw out there. For $100 you can go from 64cc to 84cc and make your saw the fastest out there for its weight.
I have run a 6400 and it runs like my 7900 with a dull chain.... lol. If I owned a 6400 it would get a BB kit immediately....
No argument there, so would I. Matter of fact I have thought about it.