There are two things that limit what the alternator will produce: thermal limits, and core saturation. When an alternator is rated at say 100A @ 14V, that is ~1400W, so no matter what you do with or without diodes, you will still be limited to 1400W. Also, if you unload the output side of the alternator and let the voltage rise, the thermal limit will still apply, ie, less current will be available at the higher voltage to avoid overheating it. Core saturation comes into play at the other end, i.e. when you load it more heavily by asking it do deliver current into a low impedance...