I believe the actual advancer on the 650 when maxed out has a total advance of around 40 degrees, if I recall what someone else posted, where the 550 is around 34 or 36 degrees. The 550 timing will run slightly cooler, the 650 timing I would think would need a slightly richer mixture, which may be why you are a good with stock 650 jetting. the more advance I believe would make more power on the top end. correct me if I'm wrong.
what was the thickness of the stock base gasket? did you have too much quench height? Or not enough? I may have this 650 head slightly milled to clean it up when I have it at the cylinder head shop getting a performance valve job.
They are maxed out on advance all the time, you almost never ride these little bikes below 3,400 RPM or whatever it is where the advance hits maximum. If you adjust the timing so the max advance is 40, it will just be a few degrees more advanced at idle, probably no big deal. You can also alter the advance mechanism so it moves a little farther, probably not that hard to get the same range as the 650 unit. Only talking a few degrees more. Slightly bigger springs will slow the advance a little should that be necessary.
I don't have the 650's ignition system, just going to go with the 550 one and see what I can get it to do.
I like points ignitions anyway, and I have a lifetime supply of points and condensers.