What clocks are you using for your 750 ti cards ?
I have 12 Asus GTX750TI-OC-2GD5 cards - default clock (1150 MHZ) and getting only 8000 jackpot coins per card. I'm mining at dwarfpool
You could OC core +135, maybe even more, but with memory be very careful. ASUS is really bad in memory OC.
I tried adding small coolers on Samsung memory, and added thermal grease, too.
But still OC vram over 200 is breaking drvier.
BTW 8-9k is fine per 750Ti card.