At this level enthusiasts are using WC as much for noise reduction as performance. I can maintain 39C GPU temps under Furmark on twin 780s (26% OC on GPU / 21% on memory) with rad fans at 1200 rpm but I limit their speeds to 850 rpm as the fans are silent till about 900 rpm. Temps rise to 44c but who cares ? The only problem is it screen goes to sleep you can't tell if the machine is on as it makes no sound.
I'm pretty sure the cap for power still exists so it's nearly impossible to gain much benefit over a well designed air-cooled card.
As for NOISE benefit perhaps on a GTX780 but this is the GTX980 that runs pretty quiet already. Add in PUMP noise as well and it may even be louder under load and will be for sure in idle usage since the PUMP has to remain on for liquid cooling but most 970/980 cards can turn off the fans for silent operation.
Where the EK water blocks stand out against the competition is in VRM and VRAM cooling where even 970 temps are well up into the 80s. Water cooling can get ya a few extra performance ticks here especially w/ memory overclocking .... I have two pumps which make no noise (2 pumps at half speed much quieter than 1 at full speed), under PWM control, it's down at about 30% speed when system is idle and top out at about 65% under Furmark w/ 2 cards in SLI.
Asus can have the two fans go off at low GPU temp and MSI can turn the two fans off independently one for GPU and on based upon several factors and while that's a significant advancement, it still does not approach water cooling..... people are finally beginning to associate CLCs with excessive noise levels. But this is by no means applicable to a custom water loop where you could be sitting right next to the machine and turn the system on and off and not be able to tell which condition is which if ya eyes are closed..
I would find it hard to justify the expense in a single gpu setup, I'm running a 2 way sli with msi 980's and the upper card when oc'd in 4k gaming pushes the temps right to the limit while the second card which gets considerably better airflow (it has 2 3000rpm scythes right below it in addition to the msi cooler) sits quite safely within temperature ranges, I'll be getting one of these to cool my top card and leaving the second one aircooled. It looks ugly but its in a case without glass panels so i don't really care.