Jeez a 100amp alternator will cover 1200watts....thats a hell of a lot of power! Maybe the AE86 had a decent sound system in it :)
On our rally car using 6 driving lights, tail lights, dash lights fan, fuel pump, heater fan, computer etc, we could only reach a maximum peak of 800 watts. We use a alternator rated at 85 amps and that is ample. In fact we used to use a 55amp alternator with no problems until we upgraded last season. These things do soak up hp and even with the kind of hp we have you certainly notice the difference when everything is turned on. A poor little A12 would certainly struggle with a whopper like 100amp!
Alternators are designed to reach maximum output at not too much above idle, so revs shouldn't be an issue as far as charging goes.
If you are using a high revving motor it actually pays to replace the standard alternator pulley with a larger pulley to slow it down. A major cause of alternator failure is due to over revving. I have never stopped to work out what revs a standard alternator would be doing when the engine is doing 8500rpm but it must be PLENTY!
When I had the A12 I used a 4" (100mm) pulley on the alternator and never had any problems.