Well I can confirm that the Magnum Mini D Lock, when flung from the bike downhill after going over a few bumps just shy of 40mph holds up well. Mine survived it's ordeal will a few deep scuffs/scrapes on the barrel lock bit. It remained firmly locked throughout the ordeal. There was no traffic behind me so I didn't find out how it fared with getting run over as well (or how well it dented car bonnets).
I've decided that it's probably best stuck in the side of the pannier for now, rather than bungied to the top of the rack. It has been fine for months, but that road is where everything quite merrily falls off the bike. So far it has been a bottle, a camera, a cheap light and now the D lock!