[aerohead]

Thanks! That's helpful.
1) The VG on the mirror is for acoustics. Noise showed up the aero-acoustic wind tunnel and the aerodynamic engineers were able to shift the resonant frequency of the mirror unit by adding the VG ( a very cheap modification to the tooling ) to kill off the offensive noise source.
2) Wheel covers won't be improved upon at any cost.
3) the forwards portion of the belly looks good up to the rear wheel region, then is compromised on account of the exhaust, heat, and carbon-monoxide.
4) Additional heat-shields could be placed strategically.
5) Fiberglass racing exhaust wrap could solve some heat flux issues.
6) Sheet aluminum would be a natural for this area.
7) The 'diffuser' isn't a 'diffuser'. It's just styling.
8) A 'real diffuser should follow where the bottom of quarter-panels project. If it can be worked out around the muffler issue.
9) Aerodynamically, with any tail extension, only the portion of the taillights, up to their burst-edges, would remain visible.
10) The existing rear spoiler would remain in place, just as it is.
11) A new tail upper would integrate to the spoiler's trailing edges.
12) New side extensions would integrate to the existing burst edges .The rear DOES have some existing boat-tailing, so you'd take advantage of that, and add to it, respecting the same contour as you have on the roof, if you can. No more. No less.
13) New diffuser would rise and extend along the path already established by the bottom of the rear quarter panels.
14) New taillights would have to be added, along with relocation of the license plate.
15) If you could relocate the backup camera, and extend the harness, you could maintain that function.
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A sophisticated and complex build! Your passport into Low-Drag Nation.