AMD couldn't provide Tahiti XTL in time so the Radeon R9 280X is currently powered by a Tahiti XT2 GPU. Tahiti XTL will come for real later (maybe november) and will be what we call a "running change", no name change, no information. This means that graphics cards powered by a Tahiti XTL GPU will be named Radeon R9 280X
and the only way to tell on which chip your Radeon R9 280X you just purchased is based on is by BIOS version. Tahiti XTL will have lower power consumption than the current Tahiti XT2. It will also be cooler.-
Advertised GPU frequencies for AMD's Radeon R9/R7 are wrong, manufacturers put in the specs only the highest GPU clock and AMD writes "Up to xxx MHz". At least nVidia (and nVidia AIC partners) had the decency to do it right writing the base clock and typical boost clock (only for nvidia cards tho).
Actual working frequencies are the following:
Radeon R9 290X: Base clock: 727 MHz / Boost clock: 1'000 MHz
Radeon R9 290: Base clock: 662 MHz / Boost clock: 947 MHz
Radeon R9 280X: Base clock: 850 MHz / Boost clock: 1'000 MHz
Radeon R9 270X: Base clock: 1'050 MHz / Boost clock: 1'000 MHz-
Boost technologies on graphics cards are either temperature-based, power-based or a combination of both (one being predominant).
The power and temperature targets/ranges may differ from a card to another (I have seen some manufacturers set higher values in the BIOS to allow more headroom).-
You can't tell what is the rev (stepping) of your GTX 780 GK110 chip via software. B1 chips are physically marked B1 but A1 is reported as chip revision in GPU-Z. B1 is correct, GPU-Z reads what the GPU says, nVidia probably didnt update the revision field that's why you see A1 in GPU-Z and not B1. If you want to know on what rev is your GTX 780 chip you have to remove the cooler and most likely clean the chip (remove the thermal paste).