I feel like this needs to be somewhere more visible, because I keep seeing people saying that you can take talents other than Momentum if you don't like the playstyle, or saying that people won't use Momentum for raiding.
From what I have seen in the current sims, even if you take Blind Fury instead of Fel Mastery and Nemesis instead of Momentum, not using Fel Rush for damage is a ~13k loss DPS in pre-raid gear. The APL for not using Fel Rush might need some work.. though I'm not sure it will make a large difference.
If you do decide to do that over using Momentum/FM and using Fel Rush for damage you're looking at a ~35k DPS loss, nearly a 15% loss in damage.
The difference between Nemesis and Momentum is actually only in the range of 2-3k, negligible at best. If the encounter is pure single target the vast majority of people will probably do more damage by taking Nemesis.
Hopefully this clears some things up, because if you had hopes of being competitive while not using Fel Rush then you're going to be a bit sad come raid time.