Yep it all comes down to how much trash at once your tank and healer can manage, and if your healer and casters can go without having to stop to drink very often.

I don't have an 80 healer yet, but I'm close. My priest is 79 and has alot of crafted epics waiting for her already when she dings. I'm currently using LFD as part of my leveling method, and if I have a good tank in the group (ie one with good mitigation/avoidance and is not a major mana sink), I can pretty much go a whole run without drinking.

So speed comes from 3 factors:

1) How much trash your tank and healer can handle at once
2) How fast your dps can burn things down
3) How little your casters/healer need to pause to mana up