Experts can't predict performance either. We're just driven by common guidelines AND THEN WE TRY OUR OPTIONS AND PROFILE.

But let's be fair - most of us can't afford test labs with multiple generations of GPUs, multiple versions of drivers, and so forth. Nevermind the time consumed in testing the different permutations. A different driver (or version) has the potential to randomly upheave the whole thing, and it's not like NV or AMD are about to help us out with real application profiles. So most of us are stuck with the handful of hardware/software configurations that are at hand, making educated guesses about the rest. I'm probably better off than most here - for my part I have access to a 6770, 7970, a GTX 480, a GTX 670, a Titan, and a couple Macbooks' worth of mobile GPUs. If this were strictly hobby work, I'd probably be lucky to even have one sample of NV and AMD available.