That's not true, and a very common misconception. Ever spin an alternator when it wasn't connected to a belt? Almost no resistance (only bearing friction). Its when the alternator is working, creating its magnetic field, that resistance increases. How strong the magnetic field, and hence the resistance, depends on the amperage being pulled from the alt at the time. If your current draw is 80 amps, it doesn't really matter if its pulling that from a stock 100amp alt or a modified 250amp alt, resistance (loss of hp) will be about the same.
300amp alts... I called Dominick to order one a while back, he expressed to me that for a daily system he recommended smaller. The problem is, the larger you go, the lower the maximum output at idle. He recommended a 260amp alt, which I bought. Im just gonna run dual alts, stocker for the truck and the 260amp for the stereo.