The name is different, D700, 7970, R9 280x, but they all use the same chip, same architecture, and same device ID.
Yes, the D700 has 6G VRAM, but it won't make it another card. The standard GTX680 has 2G VRAM, but the 4G VRAM version still the GTX680.
And if we downclock the 7970 or R9 280x to match the D700's clock speed, we should have more or less the same performance as well. So, even though they are technically different (at least the D700's PCB looks very different), they are still effectively the same card, because they use the same GPU.
The driver ident it as the FirePro, but it won't change the fact that it's the same GPU as the 7970. They can use whatever name they want. From memory, at the early stage, when people try to install the AMD driver on the nMP (of course I am talking about Windows), even the AMD software call the card "7xxx" but not D700. So, is that mean once you upgrade the software, you have a new hardware? From 7xxx "upgrade" to D700? I don't think so.
AMD rename the 7970 to R9 280X. So are they the same? They are not. But I never see any review agree that from 7970 to R9 280x is an upgrade. And almost all review I've ever read agree that they are practically the same. Rename the 7970 to D700 on the Mac Pro won't make it really have anything different from the 7970 (apart from the name).
So, I think it's all about how to define "the same". I agree that the D700 is not the same as 7970, because we cannot buy a 7970 to replace the D700. They are definitely not identical. But we know that the chip inside both cards are the same, they use the same technology, and should have the same performance (if no real world error, software differences, clock speed difference, etc). The card is difference, but the GPU is the same.