Graham, a few thoughts on The Weight Thing, from where I sit as a Raven rider. The frames of the Sherpa and Raven seem comparable in weight & geometry -- my Raven, for example, weighs 33 lbs (15 kg) unladen, with alloy fenders, F & R racks (alloy), handlebar bag mounts, 3 bottle cages, 2 accessory bars, and one tail light.
If you're keeping the Sherpa frame essentially as is, rather than (say) changing the front fork to a lighter steel or a carbon item, then the options seem to be lighter wheels, and gear.
On lighter wheels:
> I've had mixed experience here. Before my month-long ride in the Rocky Mtns in June/July 2016, I had used Mavic XM719s (shod with Schwalbe 26 x 1.6 Supremes) for a couple of years with no problems. The XM719s are mid-weight rims, 475 gms each.
> After my tour in the mountains, I noticed several micro-cracks on the rear rim, fore and aft of spoke holes, all on the non-drive side. I replaced the XM719 with a Velo Orange Escapade rim weighing 550 gms, and have had no problems since. (The XM719 on the front wheels showed no problems.) The VO rim is wider than the XM719 --28.5 mm external vs 25 mm, and the spoke holes are noticeably more offset.
On gear:
> I've invested in lighter summer-weight sleeping bag, Neo-Air thermarest mattress, and one-person tent. All these are much less bulky than the items they replaced, and that in turn has allowed me to change to Arkel 32-ltr Dauphin waterproof panniers on the rear. These are each about a pound lighter than the 42-ltr Arkels they replaced.
> On a recent 10-day mini-tour to and from Toronto, I swapped my front panniers (Arkel T28-ltr) for two 13L drybags fastened to my front rack. That gave me similar capacity, with a weight saving of 2 lbs per pannier.
> There's some loss of convenience in these changes: The Arkel T-28 and T-42 panniers have very handy and spacious external pockets which the 32 ltr waterproofs do not. And, the drybags are a lot more fiddly to mount than the Arkel T-28s. So, the drybags carry things I won't need during the day--sleeping bag, mattress, most of my clothing.
> I've augmented my carrying capacity with Revelate "Tangle" frame bags. I have a large on the Raven, and a medium on my Eclipse (my day-ride derailleur bike). Both of these are very well made, very handy, and lightweight. The Lg weighs 9 oz, the Med 7 oz. On the Raven, the frame bag carries my rain gear--jacket, pants, overgloves if it's cold, and booties--along with my clickstand. Maybe you don't need much wet/cool weather gear? If not, then with a Revelate bag and a small strap-on h'bar bag such as the Axiom Adirondack, you could dispense with the heavier Ortlieb h'bar bag & rack.
> I continue to use my small Arkel h'bar bag because it's so handy. The main compartment carries my cache battery if it's being charged, snacks, wallet, (sun)glasses, bug veil, phone, camera. The exterior mesh pockets carry things like my helmet cover for rain &/or cold, and the front pocket carries odds & ends like zip ties, sunblock, and my cleat covers. I also like the plastic envelope on top for notes, paper maps, etc. I use a small strap-on bag for day rides on my Eclipse, and that carries a surprising amount.
Hope that's helpful -- let us know how it all goes.
Cheers, John
