I think it is safer to make consistency judgements with hitters. After all, you get to see them hit about 500 times a season.
Meanwhile pitchers only make about 20-30 starts a season. It is much harder to judge a pitchers consistancy because we do not get to see them work as often.
Sure we can speculate about Peavy, but it is tough to make any concrete calls about how he will perform this season. You could have given the inconsistant label to Greg Maddux at the start of his career (due to his control issues), but you would be chewing on your words today.
Mike Pelfrey > Matt Garza