The more power that 's needed, the more load the alternator puts on the engine. The lower accelleration may be from having a weaker spark with less electrical power available for it. That can affect gas mileage adversely.
Making electricity is work. One horsepower is 750 watts and the 85 amp alternator can produce about 1200 watts of power to share with every electrical need of the car and charge the battery. The amplifier is asking for more than half the output of the alternator, but don't forget what the battery has to contribute. What the battery gives up has to come back from the alternator. You can test your actual cost in gasoline by sacrificing using the stereo for a quarter tank of gas, then refill the tank and see how many miles you got, then do that test again using the stereo. Try and do the same kind of driving. This may be a couple of 100 mile tests, depending on your tank size. Maybe you won't get rid of the stereo, but use it more conservatively if your concern about gas consumption is serious. If you get a stronger alternator, it will still ask the same load- watts is watts, no matter where you get them.