Several people in my ceramics class use a low fire porcelain clay, or at least they call it that. They get it at Aardvark, however when I looked on the website I couldn't find anything called a low fire porcelain. When I go to class next I'll ask about it.
IMHO, just a nice pure fine white clay will be equally nice at low-fire----the translucency achieved with porcelain comes with a price in the form of having to make pieces very thin, which makes them fragile and hard to work with before firing. Besides, it's more expensive!
I finally remembered to ask about it today in class. Two people have used porcelain, but it does only come in mid/high fire. One woman said she tried one bag and really couldn't tell any difference in the feel. She does have her own kiln so fired it at midrange for the glaze fire, but since she wasn't doing any pieces that were thin she went back to basic white low fire. My instructor sometimes uses it for artistic (vs. functional) pieces as she likes the way it feels, but since it's over twice the price she usually just uses the white low fire, too.