It looks like you're paying for a different cooler from the stock one. Whether it's better or worse is up to you to determine.
Unless there's physical differences (extremely unlikely in this case), the reference models reach the exact same clocks (on average) as the "upgraded" models. Overclocking is always a crapshoot, but generally you can read a few reviews and see what you might expect, on average, from a chip generation, regardless of how "supadupacl3ck3dF+\/\/" it is marketed as.
From what I remember at this hour, you can expect about +10% from the 780s with the voltage limitations and on air.
Speaking of which, fuck nvidia for limiting voltages. I'm still enlarging my anus for their dick though :-( .