While Johnstone hasn't committed too many outright howlers, I have to agree that we'd be 6+ points better off with Foster in goal. His shot-stopping is better, his command of his area is better, and his competitive edge means he wouldn't put up with some of the messing about we've been getting.
Johnstone hasn't been horrendous, but I don't think it's just a lack of experience, I just don't think he's a Premier league keeper.