Actually when you look at lens design it wouldn't surprise me ( or disappoint me ) if the new 50mm IS is actually f2 in order for Canon to make it stellar wide open.
Since nowadays it's not difficult to prevent the lens from going wide open in any mode (or sticky dof preview), I'd prefer the manufacturer not to limit the speed of a lens to get max. iq wide open and favorable review curves. The most popular issue with the 50/1.2 also doesn't seem to be that it's soft @f1.2, but that it has a focus shift.
I shoot with the EOS M and 5DmII and would love to know how to restrict the aperture range without fixing it to a particular value. Searched on the internet and did not see anything obvious on this topic. Could you link some info on this; would love to have a convenient way to have a lens shoot in its best range like f/4 to f/11 on the EF50 1.4.