I just now remembered to test speeds between the Vaio and the Mini directly. That is, entirely on the wireless net without going over the wired net. (Not that I know how that works in an AP environment: I suppose it's possible that the data's still going through the AP and a wired network problem is still causing problems.) Anyway, I just tested that throughput and it's better: about 7.4Mbps. Still not what I would expect it ought to be, but better.