[aerohead]

here's my 'two-cents' :
1) While all streamlined profiles originate from bodies of revolution, their performance is NOT strictly limited to bodies of 'circular' cross-section.
2) Streamline profiles simply provide a super-velocity deceleration ramp, free of any adverse pressure gradient, which, if otherwise present, would trigger premature flow separation, and attendant higher pressure drag.
3) The AST-III ought to perform satisfactorily for the Model X, as it's original pop-up, three-position, rear deck spoiler's tearing edge appears to intersect the imaginary contour defined by the AST-III.
4) This is true of all Tesla passenger cars, especially the generation- II Roadster.
5) Following Franz von Holzhausen's development of the Model X rear slope, one observes three 'contours' with each following contour correcting aerodynamic 'sins' of it's preceding contour.
6) The backlight 'droops' to 24-degrees, which for a 40.7% roofline, well exceeds W. A. Mair's 22-degree downslope angle maximum recommendation.
7) Aside from all US NAVY torpedoes respecting Mair's 22-degrees, one also observes that all PGA & WPGA regulation golf balls happen to demonstrate flow separation at, or very-near 22-degrees slope angle, beyond which the boundary layer throws in the towel and leaves town.
8) The Von Mises inflexion sculpted into the Model X's deck relaxes the 24-degrees, down to 19-degrees.
9) The tearing edge of the pop-up spoiler further relaxes the 19-degree departure angle of the deck down to 12-degrees, which happens to coincide with the imaginary contour of the AST-III.
10) A fully-streamlined Model X would be 294-inches in length, vs 197- inches.
11) A 24" tail extension affixed to cargo platform would net a 221-" body length.
12) Rendering a fineness-ratio of 3.31969.
13) This is MORE 'slender' than:
* IONIQ 6, Cd 0.21
* Tesla Model S Plaid, Cd 0.208
* Lucid Air, Cd 0.20
* Mercedes-Benz EQS, Cd 0.20
* Rovomobil CAD, Cd 0.196
* VW XL1, Cd 0.189
* Lightyear 0, Cd 0.175
14) The 'elongated' Model X's ultimate Cd would be contingent upon plan-taper, and diffuser geometry, however, it would reside within Cd 0.21, to Cd 0.175 territory.
15) Aero wheels would enhance the boat-tails' performance.
16) Rear skirts would also enhance the tail's performance [ Lightyear's partial rear skirts provide a Cd 0.0033 ( 1.9% ) drag reduction, and additional 15-km range ]
Full rear skirts on the SAAB 9.3 sedan generated a Cd 0.016 ( 6.42% ) drag reduction.