Hi Guys. I know that this debate between tuning vs tuning-ban always become heated, but I want to say my two cents in here, mostly because in my point of view both ideas have interesting arguments that need to be considered into a final result, and questions against that must be remembered. I hope to explain satisfactorily with this post, and find a middle-ground between you guys (which I respect a lot

).
In my [small] experience as Race Engineer before graduation, I worked in a spec-racing category (in which all the cars were the same and were cared by the category personnel). To be clear, their business model was aimed to drivers that didn't have past experience in racing, or were just going up from Kart, or just want an easy and affordable way to keep racing in Brazil (in which the racing scene is having serious problems with the Political/Economic crisis, but that is other point). In this situation, different drivers with different backgrounds and experiences were racing together. Talking with the mechanics and organizers, they said repeatedly to some of the drivers that wanted to expand tuning (only brake bias was allowed) why they kept that way. By their long experience in other categories, allowing tuning would only expand the difference between the faster or more experienced and the slowest or freshmen. Sure, It was possible that someone in the middle gain some performance, but the fastest would only increase their gap in the end. This would be the end of the category because, like I said, their business model needs people to keep their interest and motivation, feeling that they can improve and learn with the equipment, and with some chance of renewal when someone steps away. I believe that this is similar to GT Sport. If a large percentage of players keep losing interest into Sport mode, the races would starve and It wouldn't be profitable to keep the servers on.
That being said, there is the other part of the history. The categories in GT Sport, even with the BOP, allow different vehicles with different characteristics and behaviors to race together. So if the base specifications don't converge to a similar performance during a lap, some cars will always be significantly faster or slower no matter how skilled the driver is, or will have unsustainable pace due to differences in tire degradation and so on. This is noticeable in most of the Nations races, where so many people find "easier" to reach a better lap with a certain car, and sometimes the race itself turns into an one car festival (with some dude in a different car, like myself in the Maggiore Gr4 race, Final Season, a Mustang in the sea of Caymans XD). This for me kills the diversity of the grid, and if you don't like or behave well with the "META" car, well, bad for you. This could be work around with some tuning, like Praiano said. One case that I remember was the Bathurst Gr3 race in the Final Season. In a solo session and with a small tune, I managed a 2:03 lap easily with the Audi R8, BOP on. But with the stock tune the car would only reach 2:05 and I would never use the car in that race, and someone within my skill range and with the controller would never be competitive with that car (And surprise, no one in my A/S race used it).
So in the end, I believe that some sort of small tuning (Smaller range of LSD, Dampers, you name it) should be allowed to keep the races more diverse and packed, but not necessarily all open or the fastest aliens with the fastest META or with more time to spend tuning will only increase their gap and the game mode will die faster.