I believe that whilst the maximum hardware refresh rate is 50Hz, due to a technical reason it's currently at running at 48/49
This thread seemed to imply that 50 fps is now possible since Dave is now using double buffering to transfer lines to the screen:
I’m keeping the double buffer and pipelining it properly (fire off the transfer, then while that’s going prepare the next line to send once it’s done) and that gets us to an even 50 fps. Gonna call it there.
-- Dave Hayden
But it doesn't really matter, 40+ fps is great news for me 