I've actually managed to fix my re-projection/motion compensation issues. It was Window's Hardward Accelerated GPU Scheduling messing everything up. I had double, triple, burred vision in games like Elite Dangerous and Project Cars 2 with it turned on, so check you have it turned off if you're getting weird things happening at low fps.
Now everything is fine. Motion Compensation kicks in correctly and, while not as good as SteamVR's Motion Smoothing, it's not as terrible as I thought. Just the occasional artefact around edges of things - it is, for me, acceptable if not optimal. I swear Elite now runs better at the higher resolution than it did on my original PRO!!