The open - circuit voltage of an alternator is proportional to the speed and the magnetic field. At maximum revs and maximum field the output voltage will be large.
In the case of an alternator with a field coil, the field coil current is adjusted to adjust the output voltage.
Alternator power windings have considerable inductance. The result is that shorting out an alternator doesn't result in much more current than it's normal maximum output.
The impedance of that inductance is proportional to the revs, so the short-circuit current of an alternator doesn't vary much with engine revs. High revs give high open circuit voltages but correspondingly high impedances, so the current is much the same.
Alternators with permanent magnets are usually treated as constant-current supplies. On some small motorbikes and on some pushbike generators (called dynamos) the bulbs are effectively supplied at constant current, due to the open circuit voltage changing in line with the impedance.
Regulation of a permanent magnet alternator is usually done by shunting excess current. Series regulation is difficult as the voltage rises dramatically if the load is reduced at high revs.
It is possible to use a higher voltage load and get more power, but it will not work at lower revs. I did that on a 1983 Honda CG125. It came with a 25 W headlight + 5 W tail light, on 6 V, so totalling 5 A, run straight from the alternator. I changed to a 60 W 12 V headlight (and moved the tail light to the battery) so the alternator load was still 5 A. It worked fine, but was dim low revs. I really didn't mind that the headlight wasn't bright when I was stopped with the engine idling.
The OP could get more power from the alternator by running at a higher voltage, but with the revs varying, it would need some sophisticated circuitry to always get power. It would basically need a MPPT circuit that can respond very quickly, or some other way of altering the alternator output load depending on revs. It isn't an easy task for 600 W or more.