So the Gigabyte G1 Gaming GTX 1080.
After a bit of playing and a slight increase in fan profile I got 2050 Mhz stable
+80 Memory (haven't found the memory limit yet).
Note I'm on Win 7, with only 4.2 Ghz i7 2600K, lost my 4.5 Ghz to battery change, will fix at some point.
So this will probably be one of the slowest benchmarks of GTX 1080 here due to the 4 year old CPU, I doubt many 1080 will have older CPUs, maybe some i5 2500K, anyone with slower CPUs shouldn't really be running one of these.
I'll probably stick to 2025 Mhz with the default fan profile for normal use as the fan only hit's 1800 rpm and is barely audible from my case.
Summary Fire Strike Extreme:
2,050 Mhz +80 Mem, i7 2600K @ 4.2Ghz, Win 7 = 9615
1,860 Mhz (Stock OC) i7 2600K @ 4.2Ghz, Win 7 = 9380
1,860 Mhz (Stock OC) i5-6400 @ Stock, Win 7 = 8851 (test at work)
I had no problem with the PCIe 2.0, I didn't need to upgrade bios. I guess there is a small potential performance loss with this too.