I'm not really sure about this statement.
If only one T6 warlock goes shadow, he can sustain a 50% ISB uptime for 2 sp, instead of 65% with 3 shadow warlock.
that's only a ~2.73% dps nerf for 2 sp and a warlock (even if in fact the priest are less nerfed, and the warlock is more).
For 2 * 1400dps sp, it's a 80dps loss, and for a 2100 dps shadow wl, it's a 60dps loss
The 2 others walrock can go fire, which increase their dps by 5.5 to 6% (120dps for each one) if the spreadsheet is correct, and assuming they have the stuff/enchants for fire.
The total gain is 100dps.
So if you have 3 warlock, having one shadow and the others fire seem to be the most optimal way to go.
lol, what gear spec has incin 120 dps better than shadow? Like I said my calculation was 30 dps better for a gain of around 1.2%.