[RedDevil]

That's the car I would have bought instead of my Insight if I had fit in it better, and more specifically, my daughter behind me too - my chair took away her leg room...
Fifth gear is very long. You need to shift back for rapid acceleration.
You also need to rev a bit to get it going in first gear, even though it is a diesel; below turbo range there is no torque. But fuel consumption was impressive on my short test run.

I think the 'spoiler' lip actually helps.
It provides a sharp cutoff for the kammback and it raises pressure on the slightly rearwards facing end of the roof, reducing drag a bit.
Many cars end a downwards sloping plane with a flat 'spoiler', f.i. the 4th gen Prius does. It must be doing something.
I would leave it as it is.