Some interesting numbers on 4.0 BLM

[Galvuu]

snip

It's a dps loss, for many reasons.

First, something most people don't understand about how AF III and UI III interact with spells of the opposing element.
When you're in AF III and cast B3 (or UI III and cast F3), the channelling of the spell (the cast) is shortened by 50%. That means it takes 1.75 seconds.
But, on your tooltip, there's something called "recast time". That time- which is one gcd for us- is the time you must wait until you can cast another spell. That's 2.5 seconds for F3 and B3.
So even though the cast only lasts 1.75 seconds, you have to sit there for the whole gcd duration before you can cast anything again. That means that B3>F3 transition lasts 5 seconds.

You then have two other issues to deal with:
First, you forego T3. That's less procs (free pps) and also less movement options.
Second, if you get a slow mana tick on UI, you have to wait around doing nothing for 1.5 seconds on average. Or you cast a filler spell, which would be T3, this rectifying the previous issue.
Since it's extremely hard to guess a mana tick in a vacuum (aka: without seeing the previous tick, if you see one you can guestimate the next relatively accurately), you pretty much always have to cast T3/Foul to get around the mana tick issue.

The pps for your proposed rotation, btw, would be 132 (without a filler spell), which is ~13pps lower than the 3.X rotation.
With a filler spell (to get around the mana tick thing), you'd get around 136pps.
This is a 10% dps loss VS the HW rotation without a Firestarter proc.
On average, you're doing 12% less damage than a player using the 3.X rotation with a 4.0 opener would do (although it's not as bad as I thought it would be).
Whether that's acceptable or not is up to you x)