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I think you'll find that 500x is not "Recommended magnification" but rather "Maximum recommended magnification", sometimes refered to as maximum usable magnification. This is often calculated at 50x per inch of aperture, or 2x per mm - 250mm or 10" scope - 500x.
I've had a 10" dob (GSO brand), basically same focal length, never went beyond 200x with it, and in fact even with the 12" now it's rare I go beyond that. High magnifications require very clear, still air - not just at ground level where we are but up as high as there is atmosphere. We're not talking air pollution here, we're talking moisture, dust, smoke, winds, high altitude jet stream - all contribute. Often times the scope is capable of much better mag/resolution than the atmosphere will allow.
My most used eyepiece in my 10" was a 13mm - just under 100x. I had an 8mm ep, rarely used it to be honest unless I was at a nice dark site on a night with good "seeing". I always thought a 10mm would have been good.
Cheers!
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