setting your maxfpsbk to 5 will save some cpu / gpu for the main... but setting the maxfps to 999 (max'd out) would just burn the savings and make some things (ie: PiP) problematic since you have no cpu / gpu left

best to restrain your wow clients to a maxfps of 60 (do you really need more?)

personally, i'd drop it to between 40-50 and put my maxfpsbk to 10-15, just to be more playable.