The reason it looked stock is because it is stock. They get to run an open exhaust but aside from that they don't even crack open the motors. The Mazdas on the other hand, are fully built and blueprinted with all kinds of custom parts made by Mazdaspeed for those cars only.

Here's most of the tech sheets on the various WC cars:

http://www.speedvisionwc.com/competitors/vts.html