TOTALLY ABSOLUTELY AMAZINGLY WRONG!
a 4-8% loss in DPS is huge, and there is really nothing more to say about that.
There is a great deal to say about that, so lets put that into context.
Damage against Marrowgar, average of 50 guilds fights (that went on to kill the Lich King) in the last two weeks:
Warrior Fury 14,955 928
Rogue Combat 14,771 1,072
Mage Fire 13,795 854
Rogue Assn 13,783 445
Paladin Ret 13,516 705
Druid Feral 13,377 776
Mage Arcane 13,247 833
Hunter Marks 13,147 454
Warlock Affl 13,075 804
DK Frost 12,912 776
Warlock Dem 12,896 625
Shaman Enh 12,888 628
DK Unholy 12,853 668
Druid Balance 12,823 381
Shaman Elem 12,759 440
Priest Shadow 12,646 703
Warlock Destruct 12,307 630
Hunter Survival 11,721 492
Warrior Arms 11,659 567
DK Blood 11,657 777
For a feral druid, a 5% loss in DPS is a 670 DPS loss; less than the standard deviation of druid DPS. It is not a completely trivial loss, but you wouldn't be able to detect it without averaging it across multiple fights and doing some statistical analysis.
That DPS loss is 40% of the DPS loss you take by bringing a feral druid instead of a combat rogue or fury warrior. The DPS loss we are talking about is a significantly smaller than a gap that raids already accept for bringing hybrids.
With the DPS loss we are talking about, a feral would do average damage on par with demonology warlocks, enhancement shaman, unholy DKS, balance druids, elemental shamans and shadow priests. Which is to say, you are doing DPS competitive with the bulk of the DPS on a raid.
You are also somewhat better off than survival hunters, arms warriors, blood DKs ... and not even mentioning the absolute foolishness of bringing a BM hunter, frost mage, or subtlety rogue along (although many guilds do, and somehow manage to progress just fine!)
So, to come back to the point.
The loss is not 'huge'; it is a loss you should feel rather comfortable with. It is a loss to be sure, and if you are 100% focused on DPS, probably not worth paying. On the other hand, if you were trying to decide whether to swap to your 100% boomkin DPS off-spec, knowing that you can do the same damage as a feral in your tanking spec is a very valuable thing to know.
You expect some loss for having a more flexible spec. You are not going to be able to do everything as well as a highly focused specialist. You need to decide whether the loss of DPS is worth the flexibility.
From a tanking point of view, picking up the DPS talents is pretty attractive: there is almost no downside to picking up the talents to do 95% of the DPS of a focused DPS spec. From a DPS point of view, that difference is sufficiently small that you could reasonably consider using your off-spec to do something different entirely.