That's with field current right? That's higher than I would've hoped
It would be worse if that was just the bearings and belt, although if I understood correctly, your belt is less efficient than the common 6 rib belts on modern cars.
One thing to keep in mind is that the field current is adjusted downwards as the rpms go up, and your gasoline engine's efficiency is going up with increased load. Still, it's useful to see ballpark numbers.
Seems like the 0.5mm laminations on car alternators rob a fairly good amount of power. A cheapo Turnigy Rotomax 120mm only takes 200W to spin at 10000rpm despite its neodymium permanent magnets, and still not so great 0.2mm laminations, which reduce the eddy current losses by 83%.