All else being equal making the airfoil a little thicker like this should delay the stall by a slight amount.

The reality though is that you likely won't see any change at all. Especially since this is a film or fabric covered wing.

One thing I'd watch for though is to make sure you sand the cap strips so that you maintain the original point of maximum thickness. To help with that you may want to lay a strip of 1/2 or 3/4 inch masking tape along the wing centered on the original point of maximum thickness. Sand just to the edges and fair the strips off to the leading and trailing edges like you planned. When you're done pull off the tape and give the whole works a couple of more swipes with the fine sandpaper to feather in any steps left by the masking tape trick.