The mechanic talked me out of it. I'd planned to check the valves myself, but I'd have to compensate for my lack of experience by working very slow, and in my spare time. I wanted to ride some more while the good weather's here. But the mechanic was dead set against it. Said he knew all about it, that it screws up the ignition timing and would wear the engine out sooner than the stock setup. He said that to his knowledge (and mine), nobody has put more than 1-1/2 years' of riding with the mod in place, so it's untested long-term. I couldn't very well insist that he do it.
He said, If you want more power from a KLR, bore it out, but anything more than the 685 will over-stress the lower end. Jeez, what a spoilsport lol.
So come, KLRistas, talk me into doing it myself!
-JW