If 80% or so of your braking is done by the front brakes, then you are only working with the improvement on the 20% of the rear. So even if the discs were much better than drums, the gains would be minimal.
They look better than drums behind big modern wheels but with original style wheels or Cragars etc you won't really see them.
Don't you need to convert to Green bearings as well with rear discs?
I've always liked the idea of discs all round, but I've left the drums on the rear for all the above reasons.
