Alternators aren't built for one engine so the manufacturer can't possibly know the gearing / ratio between the alternator and the engine. Therefore the RPM listed in the spec will be the RPM for the alternator.
To calculate the alternators RPM you will need to find the diameter of the alternator pully [a] and the one connected to (presumably) the crankshaft [c]. Then do the following:
Ratio = [c] ÷ [a]
Alternator RPM = Engine RPM * Ratio
I have no idea how to accurately estimate power output, but I can tell you it will be probably be curved not linear.