I agree with white_n_slow. I'm running 350F, 450R with the GC/Yellow setup right now and it is pretty raw over bumpy roads, so I can't imagine bumping the rates up another 100 lbs. I also don't recommend OTS Koni Yellows for rates above 500 lb/in...you should really look into re-valved SPSS Koni's to provide proper damping for rates that high. You mentioned comfort, so you really have no business thinking about rates that high.

If you don't race, you don't have a need for custom rates and you should stick with off the shelf rates.