If I was the designer, I would also rule Cha modifier for bound weapon and +10 dmg if the -5 att from GWM is applied. Other damage bonuses it depends (flaming spear with +1d4 fire? probably not on the opposite end of the weapon. But generic magical +2? I would say yes).
Ultimately tabletop rules are relatively important considering Larian made a lot of adaptations. I also saw this topic many times, I can only infer if no patch addressed it so far that Larian intends the feat to work like it does for balance reasons.
