I mean, farm animals[barring pigs] definitely aren't sapient and aren't very bright either. That doesn't mean that they're incapable of feeling pain. Whether we should actually care about them feeling pain is one matter, but saying that they're not even aware of their own pain is beyond baffling.
I personally think that keeping animals in order to eat them sucks for them, but not as much as being permanently extinct would. And make no mistake, that's exactly what the other option is. Other people are free to disagree with me as I sit atop my chickenbone throne, munching on bacon as I pet my cat.
re: Animals not having a consciousness. There's really no reason at all to conclude that? Obviously the only consciousness you can be sure exists is your own, but I doubt even you would say that other humans are not self-aware and thus do not "truly" feel pain. Presumably you consider other humans to be conscious by examining the symptoms of your own consciousness and seeing them in other humans. The thing is, those same symptoms are present in animals too. They're capable of forming attachments, learning behaviour and even of committing suicide in the face of grief. I can see an argument for insects lacking consciousness since they're so alien to us mammals, but it's incredibly stupid to look at beings very much like us and declare that they are nothing more then meat robots simply because they're really really dumb.
FAKEEDIT:
I'd think that apoplectic rage at least would be easy to measure since it'd logically entail an apoplexy, which is something you can detect medically.