Update - I just dumped the PM differential 6 piston caliper - for a Harrison Billet-6.
Big improvement, much better braking.
Problem with the PM differential caliper is that it was BACKWARDS on my R100GS forks - it was designed to be mounted behind the forks, not in front.
Lots of discussions with Harrison while they fabricated a mount - (I have the 320mm disc on mine, so the Harrison standard R100GS bolt on doesn't work.)
Upshot of it all is:
1. Dust seals not needed, as they use a hard-anodized piston, which doesn't corrode like the usual chromed steel piston. Just give it a wash occasionally to keep the area clean, especially when riding in mud! Normally even at high mileages the pistons do not need replacing.
2. They also don't find they need a differential bore setup with the separate pads, solving that problem.
3. The easy way to go is to buy their standard bolt on Billet-6 or 4 piston version for the R100Gs or R80G/S. 306 pounds plus vat for the 6 piston, not sure the price of the 4 piston, but it's on their website http://www.billet.co.uk/
If you need a custom bracket because of an oversize disc - or whatever reason - it runs around 75 pounds, and they'll do it on the spot if you make an appointment.
Is it worth it? I'd have to say that any improvement over the standard is worth whatever it costs!
Any good 4 piston brake is way better than the standard - the R80G/S front end originally on my bike got a Performance Machine 4 piston that worked well. The 6 piston calipers of course are definitely better, but cost more. It's up to your budget, but two-up and loaded on these bikes is, in my opinion, dangerous. You owe it to yourself - or AT LEAST your passenger if you don't care about yourself, to have a decent brake.
hope that helps a little - I've had 4 different brakes on mine and they were all much better than the original.