At first, I want to clarify that I am not a nerd, but I enjoy challenges like this and want to share my experiences with you.
Since the test server is needed for other races (which are all fantastic), we couldn't collect much data. However, we drew some conclusions: the G40 is too fast, and the Abarth 124 is too slow. Finding good FWD contender is not a problem. But a RWD contender for the MX5 Cup is hard to find.
So I've tried another approach to find/evaluate good contenders for the MX5 Cup/Abarth 500 races. I set up an AI-race (for details see attached screenshot) with 10 laps. The AI-Setup 95/85 gives good racing/overtaking without cars running wild or piling up in the first corner. And I read somewhere (not sure), that an AI level above 80 enables the AI to adjust the setup or change tires according to its requirements. So I choose 95.
Regarding the grid, I tried to distribute the cars fairly (12345543211234554321). BTW: The Ginetta G40 is missing, because it was way too fast in the first attempt.
The results (see attached screenshot): The Abarth 124 is too slow as previously mentioned. The Clios might be to fast, while the Swift and the Civic appear to be suitable additions to the grid.