[aerohead]

If you wanted to 'push' the contour of the roofline to a 'radical' but 'scientifically arrived at' curvature,you could superimpose Volkswagen's latest L1 roof over the 240 SX.
This contour is from Buchheim's wind tunnel work and goes as steep as 23-degrees maximum downslope.It's 1-degree 'faster' than the 'Template' but maintains the gradual buildup of downslope.
The important thing is to respect the gentle curvature leading up to the 23 -degree tangent angle.
Anything steeper than this will be what Hucho refers to as a pseudo-Jaray form,very prone to flow separation and longitudinal vorticity.
I suspect that the slope on the Nissan is just a bit too fast.
This goes back to the argument that,cars which 'look' streamlined aren't necessarily streamlined.