The biggest gain from Exceed is the increased damage cap. You need 100 skill levels (assuming Exceed()) to reach the cap for that, increasing max burst damage from 1m to 2m.
However, you need... at best one Exceed really, because reaching 1m burst damage is not an easy task. Add one more if you have a P2W Eidolon for that element. There's some quirky Hime who will Burst really hard, so more would be fine for them, or Thunder Aphro who basically alone warrants to add one more Exceed weapon.
Any Exceeds you get more than what I mentioned above will still increase your burst damage, but it's not particularly impressive. Still, more damage is always more damage, and Wind for example has access to TONS of Exceeds, which basically gives them a free PF for every full burst. That's no joke either.
tl;dr; there is no real answer, because it depends on the element in question and what you have access to (ie. how much you whale).