I have been wanting something like this for ages (wide angle converter that's behind the lens)

That's now what the white paper says.

http://www.metabones.com/images/metabones/Speed%20Booster%20White%20Paper.pdf

They claim the performance will increase with the adapter. This is opposite of teleconverter. As a teleconverter slows the lens by one stop and decreases sharpness. This kind of converter (which was not possible before due to mirror box in DSLRs) makes the lens faster by *one stop* and increases sharpness.