In sim mode, many people tune their cars, myself included. If you detune your tuned car (powerwise), will the computer set the competition to your tuned, or detuned car? I am talking about the Pro league, if it makes a difference.
In my experience the computer takes no mind to what car you are driving to make the race fair for you. It's just random cars that all are about equal racing against you. Example: Take a PT Cruiser or Daihatsu Storia X4 out to a Pro League race and see if the computer sets you up with other Daihatsu's and Cruisers, it doesn't.