I beg to differ here on the 12" - But yes the 10" are too weak sounding. IMO the 12's are a great driver, not as much scale and weight as the 15" granted but they do have great bass and speed in the correct cabinets of course.
I've tested on 12" and 15" MG's and I still think the 12" are pretty amazing even though the 15s are the pick of the crop.
When done right (with care and attention), matching cabs and updated crossovers (especially externalised crossovers) both 12" & 15" MG's will sound superb.
Jerry, I seem to remember you enjoying some 12" Lancasters at a show once...? - Or was that someone else?