Hello Benn,
We sent the prop along with the specs to Porters, and noted that we were only getting 8.1 @ WOT 1400 revs. They took some pitch out, filled a patch of corrosion and rebalanced. I now get 8.5 @WOT 1445 revs - an extra 45 rpm, but still not near spec - I'll have a peruse of their figures and think some more.
There are three large alternators and two hydraulic pumps running off the 6lxb, plus the stabiliser fin drag, so maybe that changes the scenario. I hear what you're saying re the seal!
BTW we also replaced corroded bearings in the stabilisers