That is still 200 more premium cars than GT4. And regardless of the number of cars it's still a brand new game with plenty enough of new features to have it deserve to be called GT5. Besides, do you think you would have enjoyed the game more if it was called GT 4 part 2??
And regarding the beta statement I totally disagree as well. If you are new to gaming (or old, but haven't kept up for a while), then here is a newsflash for you: Patching is a new and modern technology to improve a game after its release. Yes, patching wasn't used in past generations of games, but that is only because the technology wasn't available at those days. All games have flaws, bugs and glitches that the producers wants to fix, and now they can! Isnt that great? And also, the kind of bugs and glitches we have seen in GT5 (so far) has not been of the beta kind. Betas are generally a lot worse in that respect.
There has still been quite an amount of bugs in GT5, but that has more to do with the game being a huge release with a lot of new concepts for a new platform. Which in itself proves my point that GT5 deserves to stand on its own rather than being a GT4.2 as you suggest. It's after all breaking a lot of new ground for the series and as GT2 was a GT1 gone perfect I think we will see GT6 as a GT5 gone perfect. I think you will have more of a case when you in 2014 claim that GT6 should be called GT5 part 2...