Jags,
My two cents' worth...
I'm not sure how your Sherpa is presently configured, but I believe you could go far in answering your question (wrt to lighter wheels vs another, more speed-oriented Thorn) by borrowing a lightweight front wheel from a cycling friend or neighbor for a trial run. Pop it in, see if you notice a difference, and use that data to determine your next course of action.
It is amazing how reduced rotating mass can make an entire bike feel more lively and responsive. A quick check on the effect can be made by swapping just the front wheel, since it is the steered one and the most easily accomplished swap. You can always get your "old" Sherpa back by reinstalling the original wheels, making the same bike more usable for touring or speed work.
On my Sherpa, major contributors to overall weigh and rotating mass are the SON28 dynohub, the Rigida Andra rims, the 36 spokes, the SV13 tubes, and rigid 26x2.0 Duremes -- all of which I specified and need for my intended use. For your use, I would suggest 32 spokes, a lightweight rim, narrow and higher-pressure tires and a conventional front hub. The ride will be much, much harsher, especially with the heavier Sherpa frame. A change in handlebar width can also alter one's perception of a bike's liveliness. Changing the headset will do little to affect overall weight or perception, so I would save there.
In the end, I agree with Oleksandr (Cedric): If you can manage it, keep the Sherpa for heavy duty and touring use, and get a second, lighter bike for other stuff. The Sherpa is a great do-all bike, but it likely won't be your fastest bike. That said, weight doesn't seem to affect my own speed or distance much, and I have often ridden my tandem solo on 200-300km day rides. The Sherpa is chasing it for weight, being only 2.7kg lighter. I do, however, own a number of lighter bikes I can choose from that better fill other needs. My primary rando bike is a lighter touring bike that weighs 3.6kg less than the Sherpa, and I will likely still choose it for 300-400km day rides in mountainous terrain. It is a matter of horses for courses, and how much of each kind of riding you intend. If you find yourself taking few loaded tours, then you probably need a lighter bike in your stable. If the need for speed is only an occasional thing, then a lighter wheelset might, indeed, fill the bill for you. I can say...it is amazing how the small hassle of swapping wheels back and forth for specific uses causes it to just not happen. One finds oneself just grabbing the bike and going with whatever is on it. That's why I have each of my bikes independently ready to go, avoiding the need to swap water bottles, computer heads, pumps, tool bags, rack packs, etc. When I want to go, I want to go with no hassle.
In the end, if you go with lighter wheels on the Sherpa, you will still have a heavy, stout frame with light wheels, which isn't the greatest combination for making speed comfortably.
Let us know what you decide, Jags.
All the best,
Dan.
