SMN BIS 2.3

[Judge_Xero]

Well doing a quick math check based on all the best gear that have spell speed and with 486 accuracy, any fights where you can use Ifrit, full spell speed (511 with food) might actually be good.
I.e. lower Crit, but more attacks from Ifrit to activate Enhanced. So ideally you stay close to 2.24GCD 10% faster casts vs the 3% damage you lose from BiS.

Casting Rotation comparing full Spell Speed BiS VS Normal BiS (Recommended using Ifrit Only)
Spell Speed Gear http://ffxiv.ariyala.com/NK7T
BiS http://ffxiv.ariyala.com/NK9D

Edited to include Fester

A: 2.34s GCD
Aetherflow, Bio, Mia, B2, Fester, SCSF, Ruin, Ruin, Ruin 2, Fester, Ruin, Bio, Ruin, Ruin2, Fester, Mia, Ruin, Ruin, Ruin, B2, SF (2.8s), B, Mia, Ruin, Ruin, Ruin, Ruin, Ruin, Ruin 2, Aetherflow, Fester, Ruin, B, Mia, B2, Fester, SCSF (at this point it repeats itself)

VS

B: 2.43 GCD

Aetherflow, B, M, B2, Fester, SCSF, Ruin, Ruin, Ruin2, Fester, B, Ruin, Ruin, M, Ruin 2, Fester, Ruin, Ruin, R2, SF, B, Ruin, Ruin, M, Ruin, Ruin, B, Aetherflow, Fester, Ruin, Ruin, Ruin, B2, Fester, SCSF (Rotation has not yet repeated itself, but it has reached the 70.66 Seconds that Rotation A repeats at)

Total Potency of Rotation A: 5240 (+1500 for Fester X 5) - Total 6740 Potency
Total Potency of Rotation B: 4860 (+1500 for Fester X 5) - Total 6360 Potency (6550 including 3% damage difference in stats)

So a difference of ~3%