It can be big numbers like this
from a specifications side, it has 50% more cuda cores, tmu's and ROP's. It has a 50% wider memory bus.
at OC, memory clocks are about 10% higher on the 770, so bandwidth is only beat by like 1.35x or so (from the wider bus) but gk110 is still a beast
most of the people reporting numbers less than like 1.4x as powerful are using stock clocks, where 770 is often at 1250mhz, and 780 ~150mhz below. If you equalize clocks, it's a much scarier story and that's made worse by the more unlocked side of gk110 - it's much more common to be able to run those cards at more than 1.212v, as basically none of the gk104 cards are voltage unlocked, hacks or not