Same Problem here using latest Omega 14.xx drivers, Steam uses 25% cpu minimum or exactly 1 core of my 4 core cpu, just sitting there, even when minimized. it will go from 25% - 28% cpu usage just sitting there, which it normally should be only using 1 - 3% just sitting there.
Oddly enough, compton, a compositing window manager uses 25% cpu minimum as well, but only when vsync'd to opengl. If I run it without vsync'ing to opengl, it doesn't even use 1% cpu.
Anything that uses Opengl seems to use an entire core no good reason.
This problem causes games such as CS:GO to stutter making it unplayable at times due to the CPU being severely crippled, because steam uses 25% cpu for no reason, CS:GO uses 25% for no reason, this leaves CS:GO only half the cpu to actually do stuff with.