The arguments about how unfair tuning would make races in sport mode are all basically not accurate or just plain false due to the way races are supposed to be matched in sport mode.
With the matchmaking first using SR, then secondly using DR to separate racers and then using lap times to set the final matches from the available online pool of racers then even if "alien tunes" actually existed and turned a driver from turning a 2:03.00 lap time into a driver now turning a 1:59.00 lap time because of this tune with the same car differences just being tunes then the law of averages says that you may have raced against this driver turning the 2:03.00 lap time because you turned a 2:03.50 but now that he is turning a 1:59.00 he is placed in a different lobby with faster racers.
Actually the tune, what car he is driving within class, whether he has a custom livery with gold plated wheels does not matter either, none of it has absolutely anything to do with which lobby he is in but his rankings and "speed" or lap time out of the currently available pool of entrants are the only factors that determine who you are matched against.
Locking tuning will just unnecessarily shorten the life span of the game for many players due to boredom and repetition of the same old thing by denying a big part of a racing games challenges.
The limited choices in the daily races, the BoP unbalance in certain classes and the lack of tuning which gives no reason to spend time developing settings that cannot be used have already started to effect the amount of time I am spending playing the game.
Eventually allowing tuning may not matter as the racing pool could possibly be void and empty of racers!
Weren't you complaining earlier about Sport mode matching lower ranked drivers with much higher ranked ones?