Hi Cogs.
The mud flaps on the rear DO make a difference.
I have them on my '88 Classic, the rear quarter panels and the rear window stay cleaner when 'on road'.
If you are really concerned about spray, fit flaps to the front and rear with the insides of H.G.V. mudflaps.
These are basically a bristle or brush construction, that not only catch the spray but all sorts of muck that gets thrown up from the road.
There is a downside though, as any Truck driver will tell you.
It takes AGES and AGES to steam clean the muck out of a Scania mud flap :!:
Cheers,
Spike.
p.s. I'm off to Cannock Chase on the 18th, so the mud-flaps will get a bending :!: :!: