A500 and A518 Parasitic loss

I can find info about the 727's parasitic loss (about 45HP). I can find info about the 904's parasitic loss (about 25HP)
But I can not seem to find any info about the parasitic loss of the A500 and the A518 overdrive transmissions.
I've read that the overdrive unit is always spinning, even before it engages into overdrive. I guess that at that point it becomes a RPM benefit .
But does the power loss (if any) out weigh the lower cruising RPM.
My current 727 is in need of a rebuild. So by installing a 904. (and pickup about 20HP). But would like an overdrive really. Would installing the overdrive suck up a lot of power ?
Also whats the parasitic loss for each, A500- and A518 ?
This swap (if made) will be in a 350 HP street drive vehicle. Might see the track once a year.