[aerohead]

I have low confidence in all desktop CFD simulations.Optimization of shape involves details which can only be evaluated in 3-D,in ground contact,at 'full-scale,' above critical Reynolds number,on software we can't afford,running on a supercomputer we also can't afford.
From your model,the nose is okay.The nose slope would be at saturation )no additional steeper angle would affect drag),and the windshield angle would also be near saturation.So far so good!
The aft-body ought to follow the 'template.'The last portion of your roof-line cannot support attached flow. The diffuser angle should be relaxed down to 2.8-4.0 degrees otherwise the flow won't follow that either.
The VW XL1 would be a good example to follow.It's dead-nuts on as far as shape goes.