You could try turning up the tone pot all the way, when I turn mine all the way up it sounds like having the 81 in the neck, so my 1 81 is essentially like having 2 That might thicken it up a bit.
I always thought of the Distortion being a rather bright pickup, like the 81, so I think the Custom would be more like the 85 if he were to go the passive direction.
I agree, the 85 will keep the agressive characteristics of the 81 but with
more bass. It won't be as edgy as the 81 but thicker. At least try it first before you gut the guitar cavity for passive pups.
Tried the 85 and liked it but I don't think it a live situation it would cut though. Chucky ass hell though. I did reocrding tests and 81 just cuts better. Just wish it was a little smoother.