Style Fan, I don't know if there's any clinical definition of "high functioning" introversion, I think it's more of a pop-psychological term, but to me it means that your introversion is not anxious. Basically, you can find a day of interacting with others to drain your energy even though the interactions were positive and enjoyable. People who are called "high functioning" introverts are often people who appear to very sociable, even outgoing, rather than the stereotype of a very withdrawn/shy wallflower. I do feel like it's a term that was pretty clearly coined by extroverts.
The question of home design and habitat is an interesting one. I don't see Angie's living room, above, as particularly "busy" despite the bright colours. I see tidiness, clean lines, nothing extraneous. It looks to me to be very orderly but also welcoming. Truly minimalist spaces rarely read as welcoming. (I often feel rather on display in very spare minimalist homes, like an exhibit in a museum.) I also know plenty of very reserved people who live in very "busy" looking spaces -- and sometimes that visual busy-ness is a direct result of leading a very vibrant private life -- lots of solitary, hands-on projects going on at all times, etc. Again, because solitude is where an introvert is at their most energetic, the place they go to get things done...
From a psychological perspective, I think social stimulus and environmental stimulus are two completely different things, and tolerance can be high for one and low for the other. Plenty of introverts live in bustling metro areas and love people-watching, etc, and in that case it's not the general degree of environmental stimulus that is overwhelming, but rather direct social interactions with individuals. I think lots of introverts rather like the feeling of being alone in a crowd, in contrast to small-town life, where no one is a stranger and you're expected to chat with your neighbours at the Post Office...