2v2 doable up to a certain extent, probably capping at around 1800, possibly a little higher if you have a good group composition. The problem though is that it is highly dependent on your class, and what you are going up against.
The main theme of 2v2 is "Survivability" most high end teams just have the ability to outlast the opposing team.

3v3 is more plausible with burst potential. I hit 1700 with my Alliance mages early on in Season 1. You are banking on that quick burst though to turn it into a manageable 3v2.
The main theme of 3v3 is "Crowd Control" in which most teams are focusing on classes such as rogues, druids, and mages in their mix to completely shut down 2 of the opposing team and finish the 3rd.

5v5 is where we shine because the main theme is "Group Coordination, and Focus Fire" a lethal combo a multi-boxer excels in. The main thing to remember here is you don't have to play 4 or 5 characters to compete in this bracket, 2 or 3 works just fine. I did very well with 3 Mages in early Season 1 almost racking up a 1900 rating and placing us close to top 30 (this is when it first started ).