I'm not totally disagreeing with you or agreeing with you either, I think the "significantly worse" is just too strong of language for me. Solo mobs don't mean much for most people, but it's what you tested it on. I think most things in GK's basic hit lands for 200 on a nudist, so lets go with that number. On a single mod for UA (18% miti)'s plus mitigation equivalent would be 36 mitigation, which is really through the roof. Damage negate is usually an option though. Miti doesnt scale nearly as well, but there are more tricks to use. The bullet version:

1) the + # mitigation mods are treated too linearly, and I think we both agree on that.
2) negate damage as an effect make the +miti builds fall into balance. This could be something put more to the forefront for re-vamping some animal builds. This effect really makes large pulls comfortable.
3) (more from my other post) I don't want whatever change to essentially nullify the crit damage change.