Not sure if the Poseidon is similar to the strix (which, if I recall, is pretty locked-down voltage wise). What voltage are you at right now? Is your ASIC high or low (use GPUz to find out).
If it seems low (read some custom bios threads to see what people are using for whatever clocks), download nvflash and the kepler/maxwell tweaker and some similar bioses. Use GPUz to save your stock bios, then use the tweaker to look at similar bioses and modify a copy of your stock bios to similar values (you can have multiple instances of the tweaker open).
Not all 980ti's clock well, but most should clock better than 1332MHz/7830MHz