Spence will never play in the secondary in the NFL. That would be a disaster, similar to the Spiller play already mentioned. Bum knee or perfect health he'd be mercifully picked on.
I'm sure Pittsburgh will use him on special teams and try to bulk him up toward inside linebacker for a year or two. Farrior wasn't much bigger when he arrived from the Jets. Taller but not heavier. Timmons is already much stronger than when they got him. Spence will likely back up Timmons and be used in spot coverage duty, like Ray Rice. That's been a problematic matchup for the Steelers on 3rd down.
Beyond anything, Canes products always turn into stronger and tougher guys once they get into the NFL. We probably won't recognize Spence's frame in a year or two...just the instincts.
I actually think Pittsburgh is a good fit, with the big maulers able to keep him free to make plays. He'll be in the spotlight on a playoff team, not a chaser on an obscure team.