I couldn't tell you off-hand what the numbers are. I've used the 645 in everything from .21 mono's up to my gas boats including the Sport 40 that I had.
You can check the spec's of all Hi-tec servos at www.hitecrcd.com .
Metal gears are less prone to breakage. Pull the cover off any servo, especially one with plastic gears and see how fine the teeth are and why you'd want metal ones in there.