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!

Someone on the french forum about mining i come from run some tests with a simple rig setup, with 2 x R9 280X

All tests ran with the same frequency etc..
Each algo can be optimized, but for comparison, it's good.

Power at the wall:

Result : Groestl is the most gpu friendly algo, for power, temp and noise, even better than X11


a little tuning on one of mine:
4 R9 280x + 1 x 7950.
3 R9 are overclocked at 1250MHz, the 4th at 1150MHz - 1.2v
The 7950 runs at 1100MHz - 1.15v
Memory for all 4 run at 1000MHz
Look at the temp (scrypt : between 75 to 83°C)