I think the banish spell and holy should have the damage potential of the equally tiered blm magics, but only be that strong when it's against undeads. The small selection of offensive spells, and the recast timers of them would already be enough to limit WHMs total damage potential compared to blm. Where BLM has several high powered nukes to switch between that they can cast with good efficiency on almost all mobs, whm would be limited to good damage only on undeads, and only when the holy and banish 3 (or 4 if we got that) cooldown was up.

Of course, to get big numbers, whm should have to get a decent nuking build, not just use the normal cure potency gear they have.