Well, depending on how much current the Arduino (and all other connected circuitry) draws, it might be enough. Or not. My position on these things is that if I don't know the numbers (and I don't, since I don't know which regulator is actually used on the Arduino) I try to find a solution for which I know the numbers. I know the PoE module is good for 1 amp, and this definitely is enough for everything.
Of course I'd love to get rid of the DC-DC converter, but until I find out how much current the regulator on the Arduino can source I'll probably stick to the converter.