I, for one, enjoy the speculation people are putting out there. Most of it seems feasible and I find it interesting.
In theory, with what we know and assuming it has a similar config to GK104, 5 SMX would have ppc between 7850 and 7870 (ala 1120 radeon cores), closer to 7850 in the way that 660ti is slightly closer to 7950 than 7870 but still pretty much directly in the center...if that makes sense.
The competition probably comes with the possible TDP. While 680 has a tdp of 190 and can be adjusted to use ~225w, 670 has a tdp of 170 and can use ~200w, 660ti has a tdp of 150 and can perhaps use 170-175. The evolution of this means 660 will probably be ~130/150 and 550ti ~110/130. With less rops and bw, which are not really needed in this market, the 192-bit chips should have more core clock room that a competing AMD part at the same tdp. 7800 series has a max tdp (powertune) of 130/170w respectively, and while that doesn't matter for 7870 (170w is essentially free-reign for overclocking) it does put an artificial limitation on 7850 that this card will probably capitalize on.
So, estimation is that it will have slightly more ipc than 7850, use similar power (130W at max overclocked), and probably clock better...maybe in the typical higher-end ~1180+ than the lower-end ~1080. It still ends up being directly between the two Pitcairn parts as 7870 has more ipc and can clock similar/higher, and 7850 has less ipc and will not clock as high, but has a little more bandwidth to even it out a little.
I agree by the time this thing launches 7800 series will be cheap...180/200/250 (or less for 7870) does sound realistic. One would hope this would launch for ~$200+ and be a better alternative to 7850, while overclocking to around 7870 performance.