Why don't you guys restrict your comments to what you actually know and have done? FYI, NONclow, the difference between modern air cooled bikes that run at CR's north of 10:1 on 87 octane and older iron that's touchier about compression lies in the design of the piston and combustion chamber. It's clear to me that you have never CC'd out a motor--increasing the bore squeezes more fuel-air mix into the same combustion chamber, RAISING the static CR. Raty, static CR is total the volume of combustion chamber (with valves closed) plus cylinder at BDC divided by combustion chamber volume. This is unaffected by cam profile in any way. In fact the measurements are best taken with the head torqued down without the camshaft in place.
The XS650 motor will run on high octane pump gas at static CR as high as 10.25:1. Higher compression will not move the power peak up in the rpm range; if you get detonation when you whack the throttle at low rpm you're in the range where you're lugging the motor anyway, and that does more damage than racking it out to redline. Raising compression is the only performance modification that will yield increased power throughout the range. Combustion efficiency will also improve.
There are downsides to increased compression. First, the engine will run hotter. This is a concern mainly if you get stuck in slow-moving traffic. Second, you need to pay close attention to ignition timing. Read Gordon Jennings' article on plug reading at
http://www.strappe.com . And of course if you wind up in the boonies where high octane fuel is unavailable, you'll need to hope you have enough left in your tank that adding just enough 87 octane and a bit of commercial booster will let you baby your motor to fuel it likes. For both of these reasons, I no longer run 10.25:1 in my motor. IMO you'll be happier with CR in the neighborhood of 9 to 9.5:1. The kit from Mike's will put you there.
Retarding valve timing will move the power peak up in the rpm range. TwoMany, with respect, your experience with deck height and valve timing may differ from mine. My belief is based on anecdotal evidence only; but a long time ago I dropped the deck on an otherwise stock motor by .030". The result was a flat spot from around 3,000 to 3,500 rpm, followed by a crazy surge of power up high that was so much fun that I left things as they were. A difference of 2* wouldn't have had such a large effect.
rv, to learn how to CC your motor and degree your cam, go to
http://www.muzzys.com. Good luck to you.