Demonic Pact is 10% SP (shares with Elemental Shamans). Mages and any other Shaman will bring the 6% SP buff.
The 4% isn't very noticeable in 10-mans as the difference in rDPS and pDPS isn't significant enough to lose out on any benefits a non-demo spec might bring (shadowfury, replenishment, cleave, etc.).
In the most approximate of circumstances, its just mathing this out. Say your destro will pump out 20k dps and your demo spec will squeeze out 18k. For ease of calculations, say both mages currently average 20k dps each. By adding another 4% SP to them, let's just assume its a straight 4% DPS increase to them both. This would equate to an extra 1,600 DPS from both of them, which is shy of your 2000 DPS difference.
But of course, you can't forget other classes that the extra SP buffs. DK's diseases, all 3 healers, both Prot Pallies' holy damage (which would mean more threat and rDPS), and lastly the often overlooked Rogue's poisons.
This would definitely offset your pDPS loss, however, if a certain boss mechanic begs for another spec, don't feel too locked in to playing demo. For example, V&T and Omno was made for Afflic or Destro due to their cleave mechanics (Soul Swap and Havoc). And Atramedes air phase screams destro's superior movement DPS.