SLI 6800's though, would provide a good approximation of 7800GTX, which was what ended up in PS3...
I dont know the history of every dev kit like I do the 360's, but I have no proof dev kits are "almost universally less powerful". Anyways, it's still case by case. If you were pushing new exotic tech like PS2 in the day, I can see weakling dev kits. If however you are shooting for something far less than current state of the art...
There is no proof as there is no known specs on any of the machines, especially PS4/XB3 which likely dont exist in firm form yet, rumors notwithstanding. I believe to know enough from Nintendo's history, shown demos (all in 720P no AA btw), reliable rumors (01.net) and public statements by Nintendo and other devs, that I believe a very accurate guesstimate is that Wii U is much closer to this gen than "next". I believe to know enough from Microsoft and Sony's history and strategy, as well as Samaritan demo, to assume they will produce a full leap worthy of being called next gen. Any such leap will be at least 10X+ based on the current state of technology (remember, 10X is a much more conservative leap when the gen is lasting 7-8 years than previous times when generations lasted 5 years).