POE2 EndGame Boss level Above +4

What happen, when you have all the 8 points of a single mechanic on their related atlas tree(for example breach) and you pick up more then 4 points to level up the boss mechanic? In breach you can have a +6 to Twisted Domain Difficulty if you decide to focus on the smaller nodes.

I'm confused what you are asking. If you have 8 points, then you can allocate up to 8 points however you wish, receiving the benefits/hinderances accordingly. You will still be limited to standard tree pathing.

You either get +6 as stated, or it's capped and you don't