[lmilcin]

Well, I decided to check the number.

20% is for uncharged, non-rotating BH. 42% (maximum) is for rotating black hole.

I have to agree with 20%. 42% assumes that you are robbing the black hole of its momentum and that cannot be sustained infinitely, as you are throwing more and more stuff into it your best possible efficiency will drop asymptotically to 20%.