One of the burning questions I've had on my mind lately has been the trade-off between Fel Mana Potions and Super Mana Potions. I have been operating under the assumption that going into the negative with my spell damage total would not affect my damage adversely, so that the Fel Mana Potion debuff could be ignored as immaterial for a Retribution Paladin. This is despite the fact that Seal of Command, Judgement of Command, Consecration, and Exorcism all have spell damage coefficients. Well, I (and the Ret Paladin community at large) might have overlooked something - operating with a negative spell damage total will decrease your damage output. Dazanna of <Limited Edition> A comrade from the Draka boards was kind enough to test this for me on some random boar. He has no spell damage on his gear. He judged Crusader on the boar and cast a Consecration, ticing for 90. He then took a fel mana pot and with the debuff it began to tic for 87. So yes, it does appear that going negative does reduce outgoing damage.
So, Fel Mana Potions still represent a boost in damage, just not as big of a boost as I originally thought. It also answers a dubious question - previously, I had thought that using Darkmoon Card: Crusade in conjunction with Fel Mana Potions was counterproductive, as the Fel Mana debuff would cancel out some of the spell damage buff from the card. This was a stark contrast to the no-risk proposition of using Fel Mana Potions and gaining the debuff when going lower than 0 spell damage was assumed to mean nothing, since getting 80 spell damage from the card meant you had something to lose. Now that I know exactly how much damage I will gain/lose from changes in my spell damage, even when going into the negative, I can safely say that the Darkmoon Card: Crusade spell damage buff loses no value when using Fel Mana Potions, even though they have inverse effects on your spell damage total. For any fight where you can ensure the DMC buffs will stay up, use it, no matter what potions you have! I'm going to go back and edit my "consumables" post to reflect this realization.
So what does this mean? For Rank IV-VI Consecration you'll lose about 24 total damage, or 3 DPS. For a max rank Exorcism you'll lose 11 damage, or .73 DPS. Seal of Command loses 5 damage and Judgement of Command is reduced by 11 ( .583 DPS and 1.2 DPS respectively). So each Fel Mana Pot reduces your outgoing DPS by roughly 5.5.
The difference is that upranking Consecration from rank II to IV (the maximum I can sustain with Super and Fel chugging respectively) is an increase of 28 DPS. So yes, Fel Mana pots will still be worth more DPS than supers still unless you somehow stack yourself to extreme levels (the debuff is capped at 10 anyway).
A comrade from the Draka boards was kind enough to test this for me on some random boar. He has no spell damage on his gear. He judged Crusader on the boar and cast a Consecration, ticing for 90. He then took a fel mana pot and with the debuff it began to tic for 87. So yes, it does appear that going negative does reduce outgoing damage.