[aerohead]

If you 'lost' that 'roof-extender' in the photo of the yellow TYPE-R, and extended the roofline down into a Kamm-back, like with the 'Panasonic WRC' car, there'd be potential for savings.
Dr. Hermann Burst, who co-developed the 'ducktail' spoiler for the 1972 Porsche 911 Carrera RS 2.7, also used on the 911 Dakar Rally car you've shared, had originally chosen a 'Kamm-back' as the solution for drag and lift, and it turns out to fit the AST-Part-C silhouette, but was shot down by committee, on account of aesthetics, and respect for 'Butzi' Porsche's 911 'falling roofline' contour.
If you did a 'Kamm-back', I'd still recommend the AST-Part-C contour, as aerodynamicists are still today choosing it for the lowest drag/lift aft-bodies. This would be predicated upon the 'lead-in' contour of the HONDA's roofline matching that of the AST.
I don't have a blueprint, so I don't know.