I'm at .62 and have had no problems. The manual claims the bike can be + or - .02 of .60.
So far as the performance / loss of mileage issue, I hear what Sidecar is saying. My mileage has dropped since adding a host of mods along with a Tuneboy and PC III. I haven't done an official math estimate, but I wouldn't think paying for the miles lost adds up to more than a couple hundred dollars a year.
And if I ever wonder if a smoother throttle, better-performing engine (especially at low to mid RPMs), and a cooler engine (especially at low to mid RPMs) is worth it a couple hundred dollars a year, I get on the bike and remind myself of the answer. :bch: