I'll second 8200 as the ideal max RPM. Over that you're high, under about 8000 and it falls on it's face during climbs.
If you're wondering about the clutch weights and springs, my experience has been that if I follow the clutching guide on the airbox, I'm usually right on the money. I also have an '04 600EFI 144.
With the 6-9k weights and springs, my engagement is just over 4500 rpm's IIRC. I'll add that my '09 M8 engages about 2500, and I like that much better!! Think I might try to re-clutch the 600 to mimic the M8... Much easier to launch w/o spinning the track a ton, which is good in the trees where I like to be!
C