I'm not sure what I think of this argument, because small differences in intelligence can have major differences in "personhood" - for example, I can't even name a number of cows it would take such that I would be equally comfortable killing X cows as one person.
I believe you - for as long as you are typing at a keyboard. Once we put it to the test and you are forced out into a field with a knife and blood pouring down your arm your 'comfort' would become far more measurable.
Even as a vegetarian, if I was forced to kill some arbitrarily large number of cows or a single human, I'd probably find a way to rationalize the cows as non-people pretty quickly. Mostly, I'd get sick of having to do the manual labor of killing them all by hand, if that's the route we're going.
http://www.smbc-comics.com/index.php?db=comics&id=2393#comic