I was chatting to AussieGhost recently about this (when attempting a carrier landing in an F-15C :-P) and he made what I thought was the good point that on the F-15 and the Su-27 for example, the speedbrake's also going to exert a downward force, opposing your flaps to some degree.
That'll mean that for an on-slope approach, if you're trying to maintain a given AoA, you'll need to have relatively higher speed to maintain it with the speedbrakes out.