Beeing an enhancement shaman in a raidguild I can speak from a frist person point of view:
You will never do real dps.period. No matter how prepared you are, with every buff available, you still struggle to make it to the top 15. Ofc you offer great support, and with a Nightfall in your hands you are hands down the best supporter class igame, but you won't be able to compete with any mage/wl/rogue/offwar that knows what he's doing. If you are fine with that and your guild offers you a raidslot, go ahead. In some singletarget fights, when you are really lucky with wf procs, you might even do ok dps, but its never gonna be great. In 5mans, geared properly, you can actualy compete with other dps classes.
Enh is really mana dependant aswell, so entering a fight with full mana helps the dps a lot.