The 'unwritten rule' for non-tank pulls in dungeons

[Awha]

Your team would have to do three times as more damage than tank+healer+2dps team, why?
Because the second team could pull three trash groups at time, while 3 dps 1 healer will struggle to go through the first group, healer will have to burn all the mana each pull and it will just not work unless you out lvl the duty horribly by 100 ilvl or so.

I was mainly talking about a tank doing single pulls, would a normal group of three dps and one healer really be that much slower then a single pull tank and 2 dps 1 healer? Granted I rarely ever queue alone so my experience is rather skewed from the norm.