of course the pump will add some power draw but not as much as what you are gaining from the lower GPU temperatures.
plenty of GPU reviews that show power draw and temperatures you'll see the correlation. a quick search shows that on a R9-290X, 20C differences can reduce the power draw by up to 30W (on load of course). Multiply that by a few graphics cards, add a CPU in the loop and you can see relevant drops in power draw.
Note that LC pumps are typically 12-15W at full speed, but you'll see if you actually get into it someday that you don't need the pump running higher than half speed - especially on a GPU.