Bow gets +1 per 5 Lv., ending at +6 at Lv. 30. CI grants upgrades in proportion to original upgrade. +0 to +5 gets additional +1, and +6 should get +2. Try using CI again.
Ah, so the bow must have been +4, which the curse upgraded to +5.
The run is over so I can't test it, but if I'm understanding this correctly, you would have to be at level 30 before using the Cursed Infusion to get it to +8?