I hope you already did the Neoclassical Challenge - the Praiano 800 PP 962 C which I used on the GTP account is now 814.23 PP, and using power to get it down under 800 PP knocks it from 600 HP to 516 PP. Somehow I doubt the AI Group C cars suffered the same fate.
On the other hand, the Praiano 800 PP Escudo with which I struggled to do the Neoclassical Challenge on the Plus account is down to 749.88 PP, and I can take the power limiters all the way off to get the HP up from 644 HP to 981 PP. Of course, if memory serves, it was a fuel hog at 644 HP, so adding over 300 HP back to the mix won't help matters.
As for the BOP changes
@tankuroded found, that's exactly the behavior of every real-world racing series that use BOP for mid-season adjustments. Of course, in the real world, it's because manufacturers and teams figure out a new configuration that's faster than everybody else, while here it's because a certain set of drivers get too used to a certain car because configurations, other than tire selection, fuel map and brake balance are fixed.