I think this bb modifier for non-core HRE provinces was made in the first place to secure the existence of the HRE and its members. When France, Poland and Austria get big, this little countries with their nearly non-existent military forces are no match and fall one by one until at about 1600 they are all part of greater realms This is absolutely not historical. Thus this penalty was made to make them last longer.
But if you now play a HRE country, the game gets rather boring, you cannot do anything without your bb going through the roof. I played Mailand and dispaired, since they have two non-core provinces at game start (which they have conquered the day the game begins, because you have to wait 50 years for a core) and a 5,5,5 ruler and a 5,5,5 heir, you will only get your bb down for long years with advisors and cardinals. Advisors are just simply luck, when there are none, you get none. And cardinals are critical, too. Your bb brings your influence on the pope down, but you need this influence to get cardinals to get the bb down...
Many small parts, making sense on their own, but together make it nearly impossible to play. And just to sit and watch, adjusting a slider and make some little decisions time after time is not very exciting. I like to take a small nation and form it into a big and mighty force others look up to. And as a german I like to play with HRE countries. But with this last patch it seems impossible.
I think, this penalty was made in the first place to deter the AI from overrunning the HRE, but now it hinders the player to take a country near that region. This penalty should at least be lowered to 0.1.