I think in the end, what enzymes do what work and how well they access the starches and proteins in the grain is what will determine fermentability. I used to think that it could happen REALLY fast, but I've since been convinced that resting a little longer to make sure all the starches have been liquified and converted makes some sense. Plus, I am not really worried about the extra 20 minutes or so I sit and wait for my mash to finish.
I do know that thicker mashes will tend to provide the enzymes some thermal insulation and allow the lower-temp enzymes (protease for example) to work a bit longer and thinner mashes will then not favor those lower-temp tolerant enzymes but be more partial to the enzymes that have greater heat tolerance.
I want to read Kai's info again...didn't he discuss this at NHC too?