I don't have a 750 TI BUT i have noticed almost across the board that those breaking 300 kh/s are doing so at +50 Core +700 Mem. This tells me possibly two things. That the new architecture is very memory dependent (as opposed to Core dependent in Kepler and lower) OR that the memory timing in the card's bios is very relaxed and a higher clock rate is needed.

I'm actually in favor of the memory timings issue as these are initial releases and mid-line cards and the scrypt algo while not reliant on memory throughput is VERY reliant on memory latency higher Mem clocks = higher scrypt hash.

This is similar to what The Stilt did for 7xxx AMDs earlier this year.

TL;DR Use +50 Core +700 (or higher) Memory and tell me what you see. You need to favor Mem over Core to stay under the power limit.