Here's Roger's review on the Metabones Speed Booster. Interesting stuff.
LensRentals.com - Metabones Magic?
Sadly, though, most EOS lenses' MF is terrible. I would much rather have one of these in an OM or FD mount. For FD, I'd certainly try my 55mm 1.2 on m43. That would take it from (eq) 110 2.4 to 77mm (eq) I think f/1.7 eq (thereabouts)?
Although a Rokinon 85/1.4 would act like 121/2.0 on m43.