There's nothing that would lead me to believe that the guy would be able to transfer in, learn an offense DRASTICALLY different from the one he came from, and be immediately effective. If he could and wanted to transfer in knowing he'd be the backup QB, I'll take him, but you know he wouldn't do that.
I'm sure I'll get plenty of smart *** responses talking about how talented he is. Piss on that, I'm happy with Relf. He's a goof fit for our offense. If he fails, we'll develop our talented backup. Mustain isn't drastically better, in our offense, than anything we have currently. It's not worth the added stress of forcing him into the system and/or tweaking the system to adjust to him.