As far as I know the primary purpose for a spoiler is to add downforce for better traction, not to improve aerodynamics. I understand that passenger cars do not normally travel fast enough givin the average spoiler's size to generate any noticable downforce so spoilers are a cosmetic only item. YMMV.

There is an aerodynamic principle that allows an aerofoil have its trailing edge truncated or cut down without much negitive effect. The shape of the CRX, Prius and Insight are all based on this principle. Adding a spoiler is just going to increase drag and lower mileage IMO.