Ok well I got it and tried it. There's no lag or skipping when something simple is shown, like the loading screen in Steam VR. However in game, there is skipping. It's playable, and I'd say about 80% of the way there, but will induce nausea at an accelerated rate.
My Thunderbolt setup is on an ASRock X370 Gaming-ITX, which apparently has a controller that limits thunderbolt to 20gbs on the motherboard side, so it's not running at 40gbps potential. To be clear, I didn't use an eGPU, but rather a generic Thunderbolt 3 expansion chassis that's supposed to do 40gbps.
I'm going to swap the motherboard and try out M.2 to PCIE. I know Vive says this doesn't work, but isn't M.2 basically PCIE (except I'd be connecting it with a slightly longer riser cable). The M.2 is supposed to do 32 gbps. Seeing as how my thunderbolt at 20gbps got almost usable results, I'm sure the other guy who did the M.2 setup was doing something wrong. Will report back in about a month when the riser arrives.