The easiest way to think of what a speedbooster does to your sensor size and focal length is the following:
When you don't use a speedbooster, you constantly multiply your focal lengths by 1.6 to get the "equivalent focal length". With a speedbooster that is no longer necessary.
WITH A SPEEDBOOSTER, YOUR APS-C CAMERA JUST BECAME A FULLFRAME CAMERA, AND ISO IS ACTUALLY TWICE AS HIGH AS WHAT THE CAMERA SAYS
So, when I use a 50mm on my NEX-5N with a speedbooster, set at f/1.4 and with ISO 200, I get an image with the same field of view, depth of field, and exposure, as I'd get with that same 50mm on a 5D3, set at f/1.4 and with ISO 400.
There, no more math. It becomes a FF camera, end of story.