I suspect that most of the marginal animal suffering arising from my eating steak comes from the production of the food used to feed the cow.

I don't know for sure, but that would be pretty surprising. I know that tractors often kill animals, but I don't think it's in sufficiently high quantity to dominate.

Regardless, as you mention, eating meat requires even more grain than eating grain directly, so the vegetarian/vegan diet still results in less animal deaths.