EDIT for better wording:
for testing, I would recommend everyone use the same agreed upon track, and all test identical cars. that way, you could post findings and it would be apples to apples comparison.
remember there is always the possibility that one car will be faster in the hands of only one racer in particular, because they understand how to alter their driving style to match it's characteristics, or because it happens to suit their driving style.
if say, the CLK is still much faster for one person, that may be that it will be their car of choice and not necessarily unfair for them to use. If it is still faster for everyone testing, then limit it further until it is relatively equal for all testers.
my .02