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

There are also things like resolution, corner performance, bokeh. Constructing an 85/1.2 lens by taking a well designed 135/2 lens and putting an adapter on the back does not sound to me as a great design. In the center, I expect the combo to work well though.

And your point is?

That is what I said, too (in terms of light gathering).