Quick Spell Speed Question

[JackFross]

So in a perfect world where this one extra cast is a Fire IV, that's a gain of ~80 DPS, yes; but to cast that extra Fire IV realistically, you need an extra Enochian segment, which is at minimum an extra four casts (Blizzard IV, Fire III, Fire IV, Blizzard III), which is currently unattainable. (Note however that if this were attainable, it would be the Fire IV DPS gain plus the extra other three spells, so much more than 80 DPS. My point here is it's something to look forward to some day, but today is not that day.)

I feel like you're missing something important in this argument. Three runs of enochian (at best) is 30+25+20 = 75 seconds. Enochian's recast is 60s. The 20s timer runs out during AF, so you will (at best) be going:
Eno wears off > B3 > T1 > F3 > Eno
Aka: 3 extra GCDs after it wears off - an additional +6 seconds. So we can assume the maximum recast for Enochian is ~80-81 seconds, and that's a slight undershot.

Spell speed would only necessitate a fourth cycle of AF under Enochian if you cut this 81 second timer (which is generally more like 70-73) down to something strictly less than 60. To shave off 10-13s from your casts in 60s, you would need nearly four to five times as much additional Spell Speed as you posited in your post to increase the gcds per 60s by 1.

Because of this massive leeway that BLM has in refreshing Enochian, Spell Speed has no soft cap with the current amounts we can get. For some actual numbers:
Assuming 0 random procs and a 2.26s GCD (since you gave me the F4 cast for that speed), your Enochian rotation would be:

F3>Eno>F1>F4>F4>F4>F3p>F4>B3>T1>B4
F3>F4>F4>F1>F4>F4>B3>T1>B4
F3>F4>F4>F1>F4>F4>B3>T1
F3>Eno

F4/B4 casts: 14*2.71s = 37.94s
GCD casts: 12*2.26s = 27.12s
Total: 39.06s + 27.96s = 65.06s

That's with not a single proc of any kind outside of the forced proc with Sharpcast for the 30s rotation and absolutely zero interruptions for movement or anything similar.
Thunder ticks for 18s. Assuming it's not flowing for the first, but is from then on, we're looking at ~11-12 ticks of Thunder which gives ~28%-31% chance of 0 procs.
You have two casts of F1. There's a 36% chance of getting 0 procs between the two of them.
Combined, there's a 10% chance that you will go through this entire rotation with literally 0 procs. Possible, but highly unlikely.

To decrease this baseline 65.06s rotation to a point where it is notably less than 60s (so 60.0 or less), you would need to increase Spell Speed to ~1460+
This drops the GCD to 2.08 (which is glorious) and causes the full rotation to be ~59.82s long with 0 procs. 10% chance that you'll wait less than 1s to pop Enochian.
Dropping below 59.0 requires 1570+ Spell Speed, and would give it ~58.64s duration with 0 procs. This would be scary, but 1 proc bumps it to 60.68, which is comfortable, and will happen 90% of the time.

So while this point exists, it's realistically unattainable in this current patch cycle.

If we attempt to maximize Spell Speed at the expense of all other stats, we still hit only a maximum of 1181 spell speed, which makes this rotation ~63s long with 0 procs.
http://ffxiv.ariyala.com/TQ67

So I dunno.

tl;dr: I think what needs to be looked at re: Spell Speed is the Enochian refresh timers, not some arguably nonsense discussion about a fourth Enochian rotation, which would never, in any realistic sense, be warranted. By the time you have the Spell Speed to do that fourth rotation, you'll also be able to hit F3>F4>F4>F1>F4>F4 in 15s, anyway, which means you'd have 4 shots of 15s AF3 rotations per 60s pretty easily.