With my r9 280x + hd 7970 I consume less power from the wall with X11 then in Groestl.

Groestl consumes an average of ~415 Watts from the wall giving 15.12Mh/s
while x11 cunsumes an average of ~370 Watts, giving 4.8Mh/s

(with optimal miner parameters for the cards and athalon II X2 270 cpu+4GB memory)
(all measured with a power consumption device plugged into the wall power socket.)

So the conclusion is X11 is more power efficient than Groestl!