The short generalized answer would be:
If you aren't threat capped and there are 2 or more people who are threat capped, max out IFF.
If you are threat capped and there are 4 or more other people who are threat capped, max out IFF.
Read on if you want to see my math behind these statements.
It's really impossible to place a simple yes or no answer on the IFF vs Subtlety debate. However, here's some estimated math to help you decide. Hopefully this isn't going to seed another pointless three page derail.
A few things to keep in mind before we begin:
If you are not threat capped, subtlety is not helping you at all.
If you are casting IFF, you lose 3.75% dps time (1.5/40)
Each point in IFF means ~ 1% higher threat cap for raiders
These calculations assume everyone is at a roughly equal DPS level
The armor reduction effect is totally ignored
Blessing of Salvation is ignored since it makes my math easier
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With this, you can already draw some easy conclusions if you are not threat capped since you only lose personal DPS when casting IFF.
With IFF1, cast it if you have 4+ raiders at the threat cap (3.75% < 4%)
With IFF2, cast it if you have 2+ raiders at the threat cap (3.75% < 4%)
With IFF3, cast it if you have 2+ raiders at the threat cap (3.75% < 6%)
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If you are at the threat cap...
Sub - # of points in Subtlety
IFF - # of points in IFF
P.DPS - Player DPS Ratio (1.0 if not casting IFF, 0.9625 if casting)
P.TM - Player Threat Multiplier (1-0.04*subtlety)
P.TC - Player's relative threat cap (1/(P.DPS*P.TM)*(1+0.01*IFF))
P.RD - Player's relative DPS ratio (P.TC*P.DPS). Yes this just cancels out P.DPS, but this is because you can still do damage to gain back the threat lost while casting IFF
Delta - Damage ratio difference from 4/5 Subtlelty
Eq1.0 - Equivalent # of threat capped players needed for an overall increase in raid DPS (Ceiling(Delta/(0.01*IFF))), No threat mods
Eq.8 - Same as above, 0.8 threat mod (Delta/(0.01*IFF)*0.8)
Eq.7 - Same as above, 0.7 threat mod (Delta/(0.01*IFF)*0.7)
Sub IFF P.DPS P.TM P.TC P.RD Delta Eq1.0 Eq.8 Eq.7
4 0 1.0000 0.84 1.19048 1.19048 0.00000 - - -
3 1 0.9625 0.88 1.19244 1.14772 0.04276 5+ 4+ 3+
2 2 0.9625 0.92 1.15189 1.10869 0.08179 5+ 4+ 3+
1 3 0.9625 0.96 1.11472 1.07292 0.11756 4+ 4+ 3+
So there you have it. In this imaginary world, if you have 4 people sitting at the threat cap(excluding yourself), then 3/3 IFF will raise raid damage. However, these do not reflect the majority of in game situations, so you'll have to make the call in the end.