The one thing that the Wilts Arch could do to centralise Wiltshire collections and make them accessible (add the argument here 'how much is Stonehenge worth?') is to relocate to the place where the visitors already go and attempt to democratise the visitor experience.
Yes, I see your point, but isn't that sort of democratising catering to the casual visitor? How many of those visitors go back to Stonehenge on a regular basis? Wot about us lot, and the more serious researcher? Do we really want to go all the way to Stonehenge every time we want to check something out, creating more traffic and more visitors at an already congested site? You can get to Devizes from most parts of Wiltshire a heck of a lot easier than you can get to Stonehenge; you can park easily there, take a stroll down to the Museum and spend a quiet couple of hours there without hoards of other people milling about.There's a bit of a similarity here with Sutton Hoo and the British Museum; the majority of artefacts discovered at Sutton Hoo are in the BM, while Sutton Hoo has the barrow itself, of course, and an excellent visitor centre with replicas and reconstructions (and a few of the original artefacts). Pity the replicas aren't in the British Museum and the originals at Sutton Hoo ;-)