No, the styrafoam will crush and smash under the weight at a high point, unlike plywood or any other kind of wood. I got this tank from glasscages.com and they even suggest AT LEAST 3/4" styrafoam underneath the tank.
For a rough example, if you lay on a bed your whole body touches because the bed conforms to your body, But if you laid on a wood floor, there are parts of your body that arent touching. With a tank you dont want that, you want the whole tank to be fully supported, or something might give, namely glass.