One thing I've toyed with (nand may do as a winter project this winter) is adding an additional layer of fibreglass to the rear, can pick up kits for boat repairs pretty cheaply, and shouldn't be a difficult job (if the yacht-types can manage it I'm sure we can!).
Only thing to u guess factor in is if you make the splitter stronger, and you attach it firmly, the next weakest point is your bumper mounting brackets, so there is a risk you'll remove your front bumper somewhat forcefully if you're not really careful!