My first 20 million cr car was either the Ferrari 330 P4 Race Car '67 or Jaguar XJ13 Race Car '66. I can't remember really, I traded for them as well as the Ford Mark IV Race Car and three of my Red Bull X2010s (although only one of them is the 20 million cr one available in the dealership). I got the Ford GT40 Race Car '69 from a ticket 1000 so I have all of the cars that are worth 20 million credits (and all of the ones worth 10 million and higher).
Have any of them proven to be superior (aside from X2010)? Yes definitely. Please note that I never drove any of them stock, only fully upgraded with racing soft tires. The XJ13 is the overall best performing one. Straight line performance is close to the Ferrari 330 P4, but it corners faster than the Ferrari and similarly to the Mark IV (which suffers from slower straight line performance, but is the easiest to drive). The GT40 is the worst one - because of traction loss on corners you'll need to bump up negative camber and perhaps LSD settings. It's the slowest one in every way.
To be honest I don't see why these historic race cars cost 20 million credits. They should be 15 million in my opinion, as well as the Chaparral 2D while the 2J and Toyota 7 should be 20 million.