I have a burstbucker 3 and a 57 classic, each in the bridge position, with A8 magnets (I haven't updated my signature line in a long time, so those changes are not reflected there).
I think it sounds great!
Going from A5 to A8 increases your output, fills in the mids, and rolls back the highs and the lows. Because you roll off both bass and treble, it can be hard to predict whether your ears will perceive it as "brighter" or not. There also tends to be a bit more sag with an A8 than with an A5, and that, plus the increase in mids, is the reason I prefer the A8 > A5.