Mostly agree with your rankings; with the optimism bias from being a cardinals fan personally, I'd argue you could make a 1 star increase at each position, but their current rankings are also fair. I very much disagree with the 1 star ranking at special teams however, as we have a pro bowl gunner in Justin Bethel being joined by some high upside athletes in the kick coverage team and kick return game. Probably the best coverage unit in the game, which coupled with a punter who is below average (don't think he's as bad as stats show- his hangtime is rediculous, and it seems the staff went with him for this reason. I'd choose a distance leg with our coverage team, butI digress). In short, I'd say 3 stars is fair. A perfectly average special teams unit, whose only limitation really seems to be Drew Butler's distance and the uncertainty of a new long snapper (but both seem pretty reliable this far)
Summary: Marcell Dareus might wind up being the best defensive lineman in a draft crop littered with top talent. Coming out of Nick Saban's 3-4 defense, he is ready for the NFL although he only started four games in 2009 and was suspended for two in 2010 so he is light on experience. His family life has been hard and teams will have to be sure he is still motivated to work hard after getting paid. Assuming he clears that hurdle, teams can either use him as a 3-4 end or possibly a nose tackle.