How much are you off at, say, 100 yds?

Shimming a tang sight is only viable because of the distance between the eyepiece and the pivot point, in that a very small shim will produce meaningful movement 2" up in the air. Shimming the rear sight base will have little to no effect, unless you are shooting at extreme range with the slide near the top of the leaf.

I would try slightly bending the front sight blade. A VERY small tweak will hardly be noticeable, yet will make a significant difference out at the target. Or, you could make up a new sight blade from thicker stock, filed into a 'lightning strike' Z profile, where it still looks "square" yet is set over a bit. Remember that the front sight wants to be moved OPPOSITE the direction you wish to bullets to move. Shoots left - offset the blade to the left.