The dark coin wiki says this about it.

Is X11 cooler in GPUs because it's inefficient?

Not necessarily. Bitcoin mining in GPUs (SHA256) requires less energy than Scrypt and yet no-one claims that SHA256 implementation is inefficient – it's just that there are differences in how each hashing algorithm uses the hardware.

Regarding X11, apparently the “gaps” in the chaining sequence, as one algorithm is hashed with the next algorithm, provide for a “breathing” space or “lag”. An alternative explanation is that certain algorithms (out of the 11) are just slow when processed by a GPU and thus slow down the whole process. The hashing of a chain can only go as fast as the slowest algorithm of the chain.

There should be some level of optimization possible in some of the hashing algorithms that increase the hashrate and energy consumption in future versions of the GPU miners. Groestl in particular is considered to be improvable.

http://wiki.darkcoin.eu/wiki/FAQ#Is_X11_cooler_in_GPUs_because_it.27s_inefficient.3F