Actually if a chunk of code that is being repeatedly executed over and over just happens to fit in 6MB then of course you would see this performance difference. They keep upping the cache on chips for a reason...
True. But I have a hard time believing that 2mb of L2 cache could make such a significant difference in Premiere Pro. That said, I don't know enough of the application or field to really say. It's important to note that while it's only 2mb more, it is a 25% increase, that coupled with the slightly higher clock and turbo boost may plausibly make a difference in a small subset of applications. Another possibility is a bug of some sort, but if the OP is really using these machines professionally for the last year then I would have to take the benefit of the doubt and trust that s/he knows what they're talking about.
I think his point was that such an upgrade could provide a tangible benefit in specific circumstances, and thus justify the cost increase for those willing to spend on performance in these applications. However it will almost certainly never make a significant difference in general use and most intensive apps like gaming. As always, value is relative.
In any case, this is off-topic, and while an interesting debate it should be left alone or continued elsewhere.
On-Topic:
I think the 650m is a relatively fast card given the constraints here. As far as I can tell this is a 45 watt TDP card, while I can't find the TDP for the AMD 6750m used previously I think before that Apple were much more frugal with TDP on discrete cards. The 9600m GT (which I'm currently using) and 330m etc were much lower TDP, on the order of 25 - 30 watts I believe (don't quote me on that, however). Combined with the i7 which is also a 45 watt TDP part that is ≈90 watts, which is insane not only for a MacBook Pro but more significantly for any notebook of this size and weight. In comparison, Razer's Blade "First true Gaming notebook" is a similar thickness (but larger overall 17") and has a previous generation card that is slower.
Maybe this will change if it gets updated to Kepler, but the point being that MBP's have never had particularly strong graphics performance (with regards to gaming). That began to change last gen, and I'm happy to see Apple continuing the trend here, despite the larger constraints. A 650m is a mid range card yes, but top of the mid range, and a competent performer. It's important to remember that Alienware's m14x (closest competitor) gaming notebook has this card too, in the same 1gb GDDR5 config no less, I think that's a first for Apple. And yet that notebook is over twice the thickness!
This has all the hallmarks of a great performing mobile graphics card, if not the very best. There's definitely signs here that Apple truly care about graphics performance (GDDR5 vs DDR3 for example). The fact that this is able to run a modern 3D game at full 2880x1800 resolution (under OS X as well, which has always traditionally had a performance hit vs Windows for games) at anything approaching playable framerates is remarkable, not only for Apple but for the industry. Yes we won't be playing at anything like that res just yet for most games, but that's where we're heading. As it stands before comprehensive reviews (hurry up Anand!), this looks like a fantastic push forwards in many ways. Today MBP enthusiasts who happen to enjoy the occasional game can get at the very least similar performance to more traditional PC's, perhaps even equivalent perf to certain gaming laptops like the m14x. I can't help but find that exciting.