Try tweaking soft shadows setting to 'soft', adjusting traffic density and variety, and turning distance scaling down to about 1/2.
Some people found their games run better with -disablehyperthreading and setting GTAV.exe's priority to high in the task manager.
A lot of the weird shadows and graphics pop-in weren't as bad in older versions in the game (v350).
Otherwise visual mods can also improve the game.