jags,
[I'm back! As it happens, my ISP -- the largest in the country -- was blocking the Thorn domain nationwide, resulting in my outage. I got their blockage reversed, thankfully.]
Back to your Sherpa and wheel dilemma; if you don't mind, I'll think out loud here and maybe we can see where you are and figure out the best way to get you where you want to go....
jags, I understand your dilemma. You're thrilled with your Sherpa and how it rides and handles a load, but wish it had more lively handling and climbing characteristics like your other road bikes. This is a reasonable desire, and something that many of us have faced in trying to make a single primary bike serve a broad range of uses. The ideal would certainly be a bike for each purpose but whole entire bicycles cost a lot. I actually think you're going about it in a better way by trying to make a heavy-duty bike "ride lighter" rather than making a light bike into an occasional heavy-tourer. The latter just doesn't work very well and can be a nightmare when loaded with more than minimum gear.
That said, I don't think the Sherpa will ever perform exactly like a dedicated road bike. In corresponding with you, I understand you realize that too, and will be happy with a heavier-duty all-'rounder that is more lively. The Sherpa may come a bit closer to what you want with lighter wheels and tires, and is worth a try, I suppose. The lighter wheels should make the bike feel more lively while the fork will simply make it lighter and (hopefully) absorb a bit more of the increased vibration you'll get with the skinner, higher-pressure tires. I just don't want you to invest a lot and then feel disappointment if it doesn't meet expectations, so that's why I suggested first trying the lightweight tire-and-tube approach first. If that make a positive difference, then you're definitely on the right track and can just swap the tires onto the new wheels. Otherwise, as Jim said, the money spent on wheels and fork could go a ways toward a second-hand bike better suited to your roadie needs -- but you want to hang onto the Sherpa.
With those preliminaries and cautions out of the way, given the way you have your lovely Sherpa already equipped with a largely road-bike orientation, may I support you by suggesting a good set of lightweight cross-country wheels? Perhaps something like Thorn offer optionally for the Sherpa -- a nice set of DT Swiss XR 425 rims, with 14/15g Sapim, DT, or Wheelsmith spokes. For the minor weight difference, I believe I would go with 36 hole rims and hubs, but you may prefer 32 if weight is a big concern. If you're serious about making the package as light as possible, then I would suggest not getting a Schmidt dynohub (they're wonderful, but heavy) and going with something like a nice set of Shimano XT, Hope, or Phil Wood hubs. If you decide you want to power gadgets and have great lighting, then the Schmidt dynohub is a nice way to go, though Shimano make a nice dynohub as well. If you go with a set of wheels like I mentioned above, they could be easily resold if they don't work out. Meanwhile, you'll still have the old kit in case you wish to take a loaded tour. If it works, you've got a more versatile bike. If it doesn't...well, you'll have given it a try and will know more. I just wish there was a way for you to borrow a set of lightweight wheels to try for one ride first. I think that would tell you right away if this is the route you'd like to go. Is there a possibility of borrowing some from a LBS with the offer of renting them or buying a pint or perhaps a round for the shop lads? They might go for it if you also committed to buying the wheels from them if the trial gave a good result.
I'm happy to talk more with you about this, jags. It is an intriguing experiment, and will be fun to see how it comes out in the end. I've enjoyed the discussion and learning more about what you want. Hopefully, you'll have the happy result you're seeking. I hope this helps.
All the best,
Dan.
