I agree completely with Bishop. Your Black Coach shouldn't be careening into combat against a Steadfast unit, or anything with a lot of static CR for that matter, unless you have a very good reason to do so. They excel as flankers, and against monstrous infantry like ogres they can inflict damage and completely immobilize them as well (provided of course that they're ethereal). I typically don't take one because I like to run magic-heavy, but it's a personal choice. I'm sure that it would perform well if I decided to bring it in a more appropriate army list.
And regarding Impact Hits, I think they almost have to count for CR. Chariots would be literally useless without them, not to mention the Steam Tank. On the charge, the only damage it can actually inflict comes in the form of impact hits (D3 per steam point used to close the gap). If impact hits didn't count towards CR, it would virtually never break anything on the charge, save for a few piddly "grinding down" hits with leftover SP.