Jim,
That's a tough one. I don't think a definitive answer can be given without alot more information, like transmission type, rear-end type, gear
oil you are using in both, wheel bearing preload, etc. Also, the horsepower loss should be related to speed (not linearly either), so a "percentage" isn't real meaningful. Probably better to think of it in terms of 40 hp drivetrain loss at 50mph , or something like that. I think 25 to 30% sounds too high for your setup. I wouldn't think that you have 100 hp or more in parasitic losses (at any speed). I know that your Tremec transmission recommends automatic transmission fluid to help reduce those parasitic losses.