Meta gems are crucial to cat DPS. You really want a Relentless Earthsiege Diamond; not so much for the 21 agility (although it's nice), but because 3% increased critical damage is smoking. Cats should crit roughly half the time, so it's roughly a 1.5% bonus to your damage output (a bit less since DoTs don't crit). And a percentage bonus scales great as your DPS improves. Right now I'm pulling about 3000 dps, so that one bonus is good for 45 DPS. When I improve my skills and get to 4000 DPS, it'll be 60 DPS. And it'll just keep getting better.
I once made the mistake of taking a helmet without a meta socket. It's almost always the wrong thing, no matter how good the stats. Unfortunately outside of PvP, there are only
8 7 feral DPS helms with meta sockets in Wrath of the Lich King. Four of the five epics drop in Naxxramas; the fifth epic is from Engineering. Then there's heroic drops from Oculus and Violet Hold and finally one option from Loken. Sadly, not a single quest reward. There are some PvP options that can be bought with Emblems, though.