It seems like (sincere intentions of actual scientists aside) the question 'do they feel pain, what kind, and when?' is so hopelessly being used as a proxy for "Can we call them Precious Babies yet?" as to be rendered almost irrelevant.
If that were the real question, it'd just be a matter of 'Well, OK, at what point do we need to use anesthetics (or a captive-bolt stunner), and at what point is that unnecessary?' We, um, aren't exactly strangers to doing potentially painful things to moral nonpersons in the interests of moral persons (or to moral persons in their own interests, or in the suitably weighty interests of others), and have comparatively loose restrictions(more so in agriculture, less so in the sciences) that mostly boil down to 'you aren't supposed to be cruel for the sake of it; but we don't actually care enough to dedicate substantial resources to the problem'. It would be a fairly arcane matter for specialty anesthesiologists and people interested in developmental biology.
As it is, of course, it's largely a proxy-war in the moral personhood fight, which is orthogonal to pain perception: Nobody says that people with 'Congenital Insensitivity to Pain' (despite that being what it says on the tin) are therefore not persons; and we similarly consider various organisms with significant pain responses to be non-persons, although they may enjoy limited protections in terms of what you need to do to avoid hurting them in the course of business.