Cree and I were able to do much of Hammer Station (the second flashpoint) at level with 2+2, but not the final boss. The content up to that part wasn't quick or easy; in particular, we took a lot of tries to get past the first boss. The usual failure mode is that companions stand in stuff, making it too hard to keep them alive, and then you start falling behind on DPS checks.
I imagine it only gets harder to use companions as you progress through flashpoints, but I don't have any experience to back that up. Being overlevel would, of course, make things easier.