level 4 weapons: Versus a +4 of its kind a +8 (since these are common for CK anyway) will do roughly +28 damage and will also receive an overupgrading bonus of +1~56. So it can be said that the difference in damage is 29~84 (or an average of roughly 56)
There isn't really a formula all I know is hard based level 1 weapons gain +2 for every upgrade, level 2 gain +3, level 3 +5, and level 4 +7
and after those base figures you add the fluctuating bonus that overupped weapons receive.
The difference in damage between a +0 CK and a +10 is 71~154
