908SSP built one. I think Gmitch has one. But no mass production exists to my knowledge. Not to mention, there is a ton of other stuff you need to be looking at before that. Vorshlag has some functional aero going on.
Agreed. So much "lower hanging fruit" exists on an S197 with respect to aero that it is almost incomprehensible to me to focus on making a Mustang a flat-bottom car (or a tunnel car). Under car aero matters, of course, but the degree to which it matters is VERY low compared to the top of the car, splitters, wings, tire spats, diffusers and other Big Ticket Items.
One of the big complaints I have with under-car aero is how easily it tends to get trashed when going off track. Sure, some cars are built with flat bottoms (Corvettes, Ferrarris, Vipers) but when you are adding this to non-flat cars it tends to be lightweight panels that hang fairly low. I've seen these destroyed on even simple offs before. If they are strong enough to take a hit they usually weigh a lot, too.
http://thinkfastengineering.com/2013/07/the-zones/
This article linked above about "production car aero" written by race engineer and aero guru Neil Roberts covers the zones of a production car where the biggest gains can be had. Sometimes it is obvious -the leading edge of the bumper or trailing edges of wheel openings - and other times it is non-obvious (places like the A-pillar are critical). Start at page 60 and read to page 65 in the monthly
NASA Speed News issue it was published in.
http://thinkfastengineering.com/2013/05/splitters-spoilers-and-wings/
This is another great article Neil wrote about splitters, spoilers and wings. Super important. This one starts on page 34 of another
NASA Speed News issue.
I recommend printing both of those articles out and reading them through many many times. Then pick up Neil's book
ThinkFAST and read it cover to cover
.