Except it is almost useless for this purpose without some serious give by the DM.
First, a warhorse, by the rules, can't be a wild cohort. A heavy horse can be though. One could argue if you could train the horse for war (via the rules in the Monster Manual II), but I'd call that a stretch.
Secondly, assuming you allowed a warhorse as a druid companion, I think most DMs would give the druid a -3 level adjustment (which seems about right). With the wild cohort feat, you couldn't take it until 7th level. At that point it would _still_ have +0HD. At 9th it would have +1 HD, and at 12th it would have +3HD. Not enough to be useful.
If you allow a warhorse as a level 1 druid companion things change quite a bit. But I'd also argue it is overpowered. Certainly better than any other 1st level companion, and as good as some of the -3 adjustment ones.