Yes it does need too be enabled both sides too be affective....
Station-psuedobridge, will probably need client mode, however i just make a habit of enabling "AP And client mode", thus accommodating for both cases.
I have found that this makes a HUGE difference in noisy areas, but yes you're CPU Usage is directly related to how noisy you're area is and how many clients you have.
This is a 3.2Ghz PC and look at the CPU Usage, this is all due too the noise in that area!
So you will have to investigate if you're routerboard has the beef too cope with the noise in that area if it is noisy!