I didn't want to go into this level of detail in the talk, but... I think you still want the MMU enabled, just not used for process isolation. With virtual memory totally disabled, a 1 GB malloc takes 1 GB physical memory even if it's not touched, you can't have swap at all, memory fragmentation kills you dead, etc. It still has a lot of utility outside of isolation.
I don't have a good sense of how the performance cost of hardware isolation breaks down into {virtual memory enabled,TLB thrashing,protection ring switching}. That's one of the reasons that I reduced the speed-up from "25-33%" in the MSR paper down to 20% in METAL. Maybe the speed-up would be less than that if virtual memory were still enabled.
Unfortunately, that distinction may have been blurred in the talk. That is, I may have implied that METAL would turn the MMU off entirely. If so, it was an oversight. I've done the talk end-to-end at least fifty times, which is how I smooth my execution out. Occasionally it can "smooth" the ideas out a bit too, leading to small inaccuracies. It's sort of like playing the telephone game with yourself (which is a very strange experience).
The MSR paper that I quote came from the Singularity team, so your reference is right on. Reading "Deconstructing Process Isolation" in fall of 2012 was probably the germ of the core narrative of the talk.