If your talking about adding another 280mm rad (for 2X 980ti's I would suggest a 360mm at the least) to the original then I would strongly question the pumps strength, (GPM) more rad and tubing adds more resistance.
If your not adding another rad then 100% no the performance is not much better than high end air like a D15 or R1.
http://tpucdn.com/reviews/Fractal_Design/Kelvin_S24/images/CPU_OC_typical_a.gif
The worst thing about what your talking about is if the pump goes out your PC is 100% down till it can be replaced. That is the major draw back to 100% water cooling and why I usually only recommend a duel pump setup for that, if one pump goes out your still up till the replacement can be installed.