How much lift is lost by wing dihedral, and can it be fixed?
The amount of lift lost to dihedral is because of a trigonometrical condition, and cannot be fixed by ribs.

Lift is always perpendicular to the local surface, and each wing half develops lift at the dihedral angle. Only the vertical bit is useful for suspending the aeroplane weight.

In order to quantify: 


Cos 5° = 0.9962, so less than 0.4% of lift is not useful.  
Cos 10° = 0.9848, meaning a 1.5 % loss.


Wing sweep has the same roll/sideslip stabilising effect as dihedral does (this answer), so less or no dihedral is required if the wing is swept. But there are many other considerations related to lift and drag with a swept wing.