Generally speaking, above some critical reynolds number, thickening an airfoil about its mean camber line increases drag over the airfoil's useful range of angles of attack and also increases the range of useful coefficients of lift. I doubt that the relationship between thickness and coefficient of lift range is proportional in general but may be so in a particular case at a particular reynolds number. As the reynolds number decrease beyond some value, increasing the thickness decreases the useful range of coefficients of lift.
Generally speaking, increasing the camber of the mean line makes the zero lift angle of attack more negative and increases the maximum lift coefficient.
The best way of dealing with these questions is to use XFoil to give quantitative results instead of using my qualitative rules of thumb.
