thanks for your responses!
as I stated, we only raid less than 4 hours a week in total and for now we can't choose heroic progression over normal farm, because many people still need gear from last bosses.
and yes, most likely our main problem is instability of our raiders.
a lot of people are having a break till MoP as well, so I was mostly concerned how about far we can go with raiding once MoP comes.
I personally don't mind current state, we're having fun. If even such group as we would be able to do heroics without problems, that would mean that it's just way too easy.