Is your core actually running at 774mhz in the hardware monitoring tool? I ask this because in my experience with a 65nm gtx 260, the core clock will never exceed a 2:1 ratio with the shader, no matter what. By having the shader at 1458, the max core clock would be 729mhz which is exactly half of the shader clock. Trying to raise the core clock any higher wouldn't change a thing. Riva tuner or precision will still show the clock changes but in the hardware monitor it will remain the same. Gpu-z will also show what RT or precision show, but the actual clocks are what the hardware monitoring show.
My shader clocks are 1566mhz and my core clock is 783mhz. If I raise it to 800mhz, it will remain at 783 even though it shows 800 elsewhere.
As for Bioshock I think it was the quality setting in the Nvidia control panel - I had it set to quality but adjusted the AA/AF from there however I think a lot of other options set to quality were still crippling things. I've set the Nvidia control panel to performance and bioshock runs at max res (1920x1440 I think) - all settings high plays OK though occasionally frame rate drops to 20 but this is very occasionally and it plays OK so will probably stick with that...
Just had a look at the Nvidia panel again to see what the default value is and I guess it's 'let the 3d application decide' i'll try that next.