It's difficult to evaluate the corner back situation due to the lack of playing time from Lewis and Burnett and Willie Gay's ability to get torched at will by opposing HOF quarterbacks such as Bruce Gradkowski, etc.
Were Lewis and Burnett simply rookies over their head in the Steeler defense? Or, are they simply not up to NFL snuff and one of them couldn't replace Gay? Hard to say at this point. I would rate the CB situation as a 9 simply because there isn't enoughb information to make a conclusion to the contrary (that is NFL caliber CB play).
Having a strong safety tandem is paramount to a good defense in the new NFL. Troy and Ryan are as good a duo as there is the game and it would be difficult to say the Steelers need at safety is any greater than 3 or 4. However, Troy was nicked all last year and Ryan may have issues with his health. The backups for these two positions are woefully inadequate and adding a safety (particularly one that can play both if necessary) would be a good plan.
The linebacking corps appears to be set, but Foote and Farrior are on the down side and Timmons is still learning to play inside in the Steeler defense. I would rate an ILB about a 7 and a rush OLB about a 4. A good inside linebacker at 1.18 would be a good plan as well.
The defensive line needs an infusion of youth unparalleled by any other position on the team. Hood appears to be a player, Hampton and Smith are two of the best at their positions, Kiesel is aging and Kirscke is the oldest (I think), the others are unproven. I see this as an 8 on your scale and another defensive end or a NT would be a welcomed addition as well.
And, just so Oviedo doesn't think I can't get out of the box and look at the offensive side of the ball I would rate running back a 6 or 7 and drafting Spiller at 1.18 (but no higher) would be a nice addition. Although, I'd rather see the bruiser Blount later in the draft, Spiller certainly would bolster the RB group.