Glad to hear you're getting 26% better gas mileage, but it seems doubtful that just alternator load could be causing that directly.

Just some rough numbers to calculate the ~max load on the alternator:
100Amps * 14Volts = 1.4 kW, or about 2 horsepower
So this power generated by the alternator would be drawn from the engine via the accessory belt.

That seems too small to be dragging your 300+ HP ride significantly. Right?

Maybe the reduced voltage was causing some other ignition / fuel management problem that reduced MPG?

__________________
2001 X5 3.0i, 203k miles, AT, owned since 2014