Check out the gap on the 600.1 side. It looks like it was designed to fit a 1200.1, but they scaled it down when they realized most people probably don't need that much power. The 500.4 and 1200.1 are the same size, so there should be plenty of room to have one.
When I found my Roadster earlier this year, I bought an RSD 1200.1 to mod it if I needed more power. It will fit in the Roadster sink with some modification to the 1200.1 board.
