I'm pretty sure they don't feel pain once dead
And removing all feeling so they live like a vegetable for their life span really doesn't seem to be all that good either.
The method of killing is really the only contentious issue, with the goal being to have their death as quick and painless as possible. And of course they should be treated humanely while living. So why meddle with their genetic makeup?
I will never knowingly consume
any product that's been genetically modified in any way, as I think it's very wrong to unnaturally change such things. Do we really know what we're doing, long term? It's bad enough that everything we eat is sopping with various chemicals.
Leave nature alone.
I'll edit to add that whether I'm being hypocritical or not, I would be in favour of genetically modifying the scientists that dream up these ideas.