I have very little doubt that Kuz can be a top-6 NHLer. All you had to do is watch him take it to Team Canada at the WJC, in Canada, in the finals. Big game performance, big time skill.
The main question IMO is how patient will the Caps be with him, and how he'll handle possible ice time frustration, especially early on.
Caps will be scrambling for every win, and Coach OCD will not be happy until Kuz learns the details of his "systems", which will take a while.
On the bright side, his competition is Brouwer/Laich, so it should not take a lot to stake his claim as an offensive contributor. Plus, if the Ovi/Backstrom split holds, playing with Backstrom on the 2nd line should be a big help in making the transition.