When comparing cards, simply look at the stream processors and to a lesser extent, clock speed to determine performance.

7870 = 1,280 stream processors
7950 = 1,792 stream processors
7970 = 2,048 stream processors

Right there you can get an idea the performance difference. The 7950 is usually clocked slower though too.

Anyone know what the settings to crunch 6 and 8 wu's in the app_info file? Want to try it after the stats update tonight but do it gradually to see how this card handles the heat.

In my case, my HD7770 is rated for PCIe_v3.0. However, after the machine bootup and after crunching HCC-GPUv6.56-WUs, on the next bootup at the BIOS, I see 'x4' where the BIOS used to read 'x16' (actual read) from an 'Auto' BIOS setting. So, I'm not sure if the HCC-GPUv6.56 have anything to do with that or just the BIOS reading that the GPU-load requires only an x4 mode. If I have to guess, that x4 mode seems reasonable in the sense that all the pixels that would have gone to consume the PCI bandwidth, which is typically heavy for games, should not seem to be case for a GPU-compute application. The fact that I also use my i7-3770k's integrated-GPU to handle the UI-visuals would also add to make that x4 mode seem to me more than adequate. Give or take, roughly, I guess an X8 mode should still prove more than enough considering that PCIe_v3.0 offers twice the bandwidth of PCIe_v2.0; hence: PCIe3.0 at x8 mode equals the bandwidth of PCIe_v2.0 at x16 mode.
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Clocks are actually irrelevant with the HD 7000 series since you can simply use CCC or MSIAB to set the clocks the same or higher than any other model in the family, this is because the stock frequency of all HD 7000 products are actually heavily underclocked (In example, my HD 7950 is running at 1150Mhz core frequency). There is also a new firmware for all HD 7000 products to increase their stock frequency to GHz level.

It looks like it should but I have not tested it. Be advised that there is more work involved than just dropping that app_info file into the project folder if you want to run multiple GPU and CPU tasks together. I would also recommend you update to boinc version 7.0.36 first if you want to try this.