For the shadows, they are CPU/GPU intensive. those actual simple blobs are really ok.
What could be done would be make them less transparent and more noticeable like in some Iphone games.

Great sculpt (yes sculptris rules) !
Your retopology seems very hight in terms of polygons for the low poly version ? will it be a giant fly or a big boss ? caus such level of details should be for camera very near and perhaps it won't be lot of times where the player will see it closer ?
At some distance from camera, lot of polygons won't bring definition on your model, you can optimize.
Anyway great model laugh