I don't know about bang average. Inconsistent, sure, but he's not average. Hard to say exactly what his top level can be. I wouldn't take much stock in his brief Chelsea career as I think that was more of a conflict in what Mourinho wants of out his players and De Bruyne's mediocre tackling and defensive play.
He's young version of Paul Scholes. Can pick a pass, scores some wonder goals from distance. Clearly not at Scholes top level, but Belgium doesn't breeze through it's WC qualifying group without De Bruyne's goals, assists, and general midfield play. He's not a great tackler, but he runs everywhere.
He's the type of player that can carry a team offensively for decent stretches at time, but he needs to play every game to maintain his level.
I don't think Wolfsburg is his top level. That we'll find out in 2-3 years.