I put a 14 tooth front sprocket on my XT660X ages ago, and it smooths out snatchiness loads as the bike seems to be in the correct gear more often, Yamaha certainly got the final drive ratio too tall on this bike.
As for the speedo accuracy I thought it now over-reads rather than under-reads so 32mph indicated is 30 actual and 64mph was 60 actual speed.
But I could be wrong I can't remember how I worked it out now!

cheers, strat.