He's a top 2 dman on a different team, not top 4. Players nearly always see a dramatic bump in play after leaving the Oilers, why should we not apply the same principle to Nurse?
Nurse is 10x the player J Schultz was and look how that turned out, hence why Dubas wants him. Dubie knows that Nurse will instantly be a #1 D for PIT. Their relationship also spans back over a decade, so Dubas likely knows the immense brain and dedication Nurse has for the game. Would not be surprised to see Dubas get in a bidding war with Briere and Kenny Holland, all three teams seem to covet a Nurse type player, albeit for different reasons.