I think you're looking at this from completely the wrong perspective.
It's not a case of 23% of your customers getting a lower quality experience, it's about 77% of your customers getting a better quality experience. If the new features worked in IE6, they would be used in IE6. They don't, so IE6 doesn't have them. If you make the entire site 100% IE6 compatible, you'd be hurting 77% of your customers who would no longer be able to do things that they would otherwise be able to do.
I don't really see the logic in you saying 23% of your members are getting a lower quality version of the site. They will get that version of the site, whatever. Taking out anything that's not possible in IE6 would make no difference for those people.