The importance of job points

[Alhanelem]

The issue here is that you haven't accounted for the forced delay that accompanies any magic action (3 seconds)

You can definitely cast tier 1 black magic less than 3 seconds after it finishes casting, unless I have a really funny idea of what "3 seconds" is.

So you've set up:
T1: 1600/3.1 = 516 DPS
T4: 3500/4.2 = 833 DPS

Therefore we seem to agree that T4 spells have higher DPS than Tier 1 spells, which is all that I stated.

Again, unless i've gone crazy, it takes more than 4.2 seconds to recast a tier IV spell. That also doesn't factor in the casting time, because you can't start casting until the recast is up. Even with a large amount of haste and fast cast 4.2 seconds from the first spell hitting to the second spell hitting seems way too low.