hmm, here the measurement I did with my budget 750ti gainward:
no OC: 4.2~4.3MH/s
mild OC +150/+150 ~4.7MH/s
extrem OC +200/200 ~4.85MH/s
ultra at 250/200 ~5000MH/s followed ny a crash because the card is a bit smal to handle all that.
Assuming the number you claimed rae correct, you have at most a 200kh/s differece which is still far from the 10% you are claiming