I know that Fitness doesn't affect Condition loss -- I have done carefully prepared testing. You can find it
here.
In short: I played a couple dozens of games against the same team to compare three very similar players with different Fitness to each other.
I have never cared to post the result, but it was a big fat zero. No correlation between Fitness and Condition loss in games, or it is tiny. Definitely less than anyone can noticed without having statistics.