I'm still not sure it has. There is no such thing as a 350Z 300, it's just a term that we as owners use (what is the official term? Is it a revup? DE facelift? Z33.2? I don't know the internal naming convention) and the damning bit of evidence is the V5 itself. J17 have advertised and sold the exact car that's on the V5, it's just that the V5 isn't correct but that's not down to the dealer to sort. Hey, they may have bought it in thinking it was a revup model seeing as how that's what the V5 says!
Given their history of selling Zeds I too would expect them to know the difference, but unless the OP specifically asked them if it was the 300bhp/revup model, have they actually done anything wrong? The OP viewed the car, was happy with both it and the price, and loved his car up until the point he realised he bought the normal DE.
Absolute sympathy for the guy, and without the J17 history I may even give them the benefit of the doubt, and I'll be very interested to see what they come back with.