I did a 25m OS 5 minutes after turning 80 and did just fine. I don't even think I did any normal dungeons at 80. If you aren't retarded you'll do fine in even naxx 10.

I always advocate getting geared before heroics, but so many people don't bother. You know what, its okay in my book. If your group can't cut it, they can't cut it. If the dps is bad then replace them. Ditto for healer/tank. However some classes even in level 78 blues or beyond simply don't do much damage, it really takes naxx epics to bring out the full realization of dps potential.

At 80 as a sv hunter I was doing 800 dps while leveling. Completely bad, I know. In a heroic I did about 1200 dps, still really low. After a week of improvement I'm at 1500 dps but all in 78-80 blues. What really went up for me is the hit cap and intelligence. That toon needs alot more work, but at this point there is only a handful of stuff available in heroics that are substantial upgrades (the epic ranged weapon is one).

Last night I healed a heroic OK run. The dps was doing 1000-1500 dps. We downed the bosses without a problem. Maybe the tank was good, maybe it was my healing, idk. Point being you can get away with sub-par dps if others are better geared (if you think this is being 'carried' i won't argue against you).

Anyway point being, unless you run a ton of heroics, don't expect your dps to shoot up overnight. It's pretty typical to run heroics even in crappy gear and have nothing good drop.