if you look at it as out of the box clock speeds then the % is much higher
1600 or so mhz oc'ed to 2100 is a rather large number. 31%ish
of course we know that boost 3.0 makes this number nonsense as every card boosts to 2 ghz or so anyway. reviews and such show such a small increase in fps, that it is jut not worth it to me to do it. a lot of extra power draw plus possible lessening of lifespan for a 1-2 fps increase just seems not worth it to me.