Hard to say. Matthysse has one hell of a chin. He doesn't even flinch or look ever a little affected when someone lands on him.
I think it would depend on how much power Matthysse really has. Enough to put down a prime Mosley chin? I really don't know about that. I know Matthyse would land some, but would he land enough. I think the likely outcome is a Mosley UD, but it's possible Matthysse has the power to get it done. Multiple people including Zab say they have never been hit harder.