Honestly, to get a proper BOP you would have to get separate settings for each track in the game, separate for quali and races, and also adjusted for tyre/wear multiplier. At that point the numbers become so astronomical that PD might as well program identical cars only with different bodies (basically NASCAR GT3). Everyone has equal performance then, but do you really want that?
To me the fun of Manu is learning each car's nuances, strengths and weaknesses and adapting my driving to suit. The BOP doesn't matter, because I know my skill will be the limiting factor long before I hit the limit of the car. I've seen LIGHTNING win top split races in RS01, Viper, GT-R Gr.4 and of course he won Manu in WRX last year. None of those are particularly OP cars. If you have the speed, you'll find a way to equalize the cars. You won't win every race, but over a whole season it should be close.
The key point is, PD employs alien drivers to test the BOP. Their goal is so the broadcasted top split and WT races are close. Unfortunately, this means in lower splits, some cars are harder to extract the speed out of and that's why people think there are metas and turtles. Nations Cup races, there will always be metas for a particular track. That can't be avoided. Personally I think all Nations Cup races should be one make - it should be driver vs driver and car choice shouldn't matter.
IMO Gr.3 BOP is the best from all classes.
Gr.4 BOP is reasonable given they need to balance 4 drivetrains (ideally, they would just have FR/MR).
Gr.2 BOP would be very good if they split into old & new.
Gr.1 BOP is hopeless, but that's to be expected if you lump cars from wildly different eras together.
Gr.B BOP is also very good, they just need to remove the old Audi.
N class should be scrapped. Treat each car and each race individually like Gr.X.
P.S. Assetto Competizione sets BOP for each track individually. It's all done in the background and you don't even have power/weight figures in the game. They based the numbers on what the real life teams use and there are STILL 1-2 clear metas for each track. It's just unavoidable unless you're racing a spec series.
Oh, and the Cayman GT4 is so OP, that in real life and ACC it's BOP'd by raising the front ride height higher than the rear to compromise its handling. You won't see that in GTS