No. RGIII was way too worried about his "brand" to be a good QB. Plus he didn't know how to take a hit. Every time he made contact, he got blown up. Some guys know how to slip a hit and others take it at full strength.
I think he'll get another NFL shot before he goes to the CFL.
As men, we grow up at different times. We expect athletes to be mature when they sign a pro contract. The light may come on later. But, yeah, Manziel may figure it out too late and end up incarcerated/dead. He wouldn't be the first mid-20s guy to take that route.