Metabones speedbooster: is there any reason to get full frame now?

What you’re saying holds some truth if you always use the lens wide open and not a base ISO, but even that doesn’t work quite the way you described.

You might find this to be interesting:

http://www.dxomark.com/index.php/Publications/DxOMark-Insights/F-stop-blues

http://photography-on-the.net/forum/showthread.php?t=950999

http://www.luminous-landscape.com/essays/an_open_letter_to_the_major_camera_manufacturers.shtml

It seems like you’re saying that as long as you leave the lens wide open and are not already at base ISO then you can use a lower ISO and get better results.  I can’t argue with that.  However, that seems like a very limited scenario, plus you need to take into account the issue raised in the above links when using a fast lens.  And if we’re not talking about fast lenses then what are we really gaining?

Having said all that, I’ll keep an open mind.  If somebody mounts an 85mm f/1.2 lens on a NEX and it produces images that match, when printed large, the same lens on a similar generation FF/35mm body then I’ll be impressed.  Very impressed.