It wouldn't have even had to be particularly fast. If they could have put a Mazda sports car engine in it (for example) and turned some laps at Indy, Homestead, and Mid-Ohio without looking silly, that would have eased a lot of fears among decision makers (based on Purnell's comments).
Alas, that was probably prohibitive given what it takes to build a car. As it was, they were the only proposal that even built a damn model. Man, if they had even cut out the wheel wells to nip the "but it can't turn!" calls, it would have been better. And maybe something other than battleship grey paint, huh?
It's like they hated marketing so much, they designed the project to fail.