Shumbori :
vortical :
The performance would be lower on a single card but so will the amount of headaches that come from crossfiring cards.
What do you mean
the amount of headaches?
Many games have poor crossfire support(especially early after release, or indie games) leading to bugs or framerate stuttering and often forcing you to disable crossfire altogether.
This usually isnt a problem for triple A titles especially after a patch or two but its definitely something to consider..
Source: I crossfire 2x hd7870's