To the OP, since you're obviously referring to Chaos more as the Moorcock/Gloranthan force than D&D chaos, here are some musings.
1. The gift must be "worth it". Why would one go to Chaos if the rejection outweigh the benefit of the chaotic gift? I can see someone drowning acepting gills and fins, but that's not something you'd seek just to win the swimming contest. Chaos is not necessarily repulsive. Arioch is good-looking and most chaos lord have high charisma anyway (but that's a gaming artefact). I'd say that the benefit should be guaranteed but the drawback be variable. It could be another benefit (ok, you've gills and webbed feet... but you've the face of the devil and it more than outweigh the rejection to your odd physical appearance. Have +2 CHA. Or something neutral. Or something even worse (a third eye just popped on your forehead... keep it or burn it -- it will leave marks, have -2 CHA or a third eyes that people will question and -1 to CA because you can't parry very well with the sensory input of your third eye). I'd say some of the mechanical benefits you offer are very mild for a -3 CHA penalty on social interactions. I'd push them up, or reduce the risk of drawback.
2. There might not be simultaneity between the gift and the mark that cause the social rejection. Sure, your leg become stronger right now... but two levels down the road, they will be so modified that you will no longer be able to stand up, always crouching and jumping (at the same speed...) and your gait will cause the drawback.
3. I like the idea of the social rejection being a common result, even if avoidable as it would push people to embrace Chaos after taking a "first sip" of it, while others will think they can get away with it.
4. For bonus horror, have some of the traits become inherited, as chaos tainted their very being. That's how you can "corrupt" whole races. Melniboneans got a lot of inherited bonus and the drawback of lack of empathy, they won the lottery. Tolkien elves corrupted by err... Melkor? inherited STR and CON bonus, but hefty penalties as well.