The Problem with Red Mage . . .

[Economizer]

If the spell cap could be lifted based on effective magic skill value you might see this.

404/20 = 20.2% New haste value

Next closest would be WHM @
378/20 = 18.9%

Except Light Arts instantly closes that small gap, then the top end cap of 500 Enhancing Skill hammers it into dust. I suppose you could "break" that cap, but 25% Haste at 500 skill is quite a lot.

So what would you get? Red Mage's +97 (100) in gear and then an extra 16 in merits for a maximum possible of roughly 517? I suppose you could sub Bard for Advancing March so you can cap out Dark Knights without bringing a Bard main, but the victory here is allowing Bards to drop one of their Haste songs (depending on how well geared your Bard is you'd be essentially buying 65~73 extra attack for your Party).

White Mage can get +107 (110) in gear, then an extra 16 from merits, even without Light Arts, which results in 501 skill. Basically, capable of doing the same thing but with shorter duration.

Considering the situation with Bard holding the key to all the Haste (with Scholar lockpicking it with some Embrava) this would be an interesting change for lowman, or situations where you cannot get a Bard, but I'll counter with some food for thought - most situations you have a well equipped Bard in could just as easily be served with higher tier/potency on Dia and have the same effect ultimately.

When was the last time that somebody, in trying to low-man whatever event, considered the versatile Red Mage in their group to conserve party slots?

Exactly one second before Scholar could cast Haste.