Hm... Just did some research into the press-in rectifier diodes used in the rectifier assembly. Apparently, they have a forward bias of around 1 VDC, instead of 0.6 VDC.
SG-9LCNR datasheet [pdf] SG-9LCNR datasheets manu [SANKEN] pdf datenblatt ,view PDF - - WWW.ICPDF.COM
That really changes things. Re-running the numbers, using both this new information, and the fact that my alternator uses 12 diodes to begin with, gives a more stark difference in thrown away power.
At a current draw of only 35 A, my alternator currently wastes about 210 W through the diodes, or a bit over .28 HP. At a current draw that I figure to be about 90 A for what I would normally run, my alternator wastes 540 W through the diodes, or 0.7 HP. At the rated output of 136 A, my alternator would waste 816 W through the diodes, or 1.09 HP.
With installing this modification (and presuming it works), I would see those numbers drop dramatically. At 35 A, my alternator would waste a total of 7 W (0.01 HP) through the MOSFET circuits. At 90 A, my alternator would waste 47 W (0.06 HP) through the MOSFET circuits. At the rated 136 A, my alternator would waste 108 W (0.14 HP) through the MOSFET circuits.
All I can say is... wow.