I think we need more pictures.
As you can see, if you took the windshield's curvature and extended it a few inches to the side of the car, that's about where Porsche's mirrors start, the shape is like a "bullet" that goes back and is cut off at the place where the driver expects to see a mirror out the window.
Compare to the new Corolla:
A blunt, flat angled rectangle shape. A few inches shorter overall.
I wonder, how might one add length to the front of that Porsche mirror without increasing interference drag? Maybe extend the "bullet nose" forward and curve it in a little?
The Lexus LFA has really wild looking mirrors and they claimed it helps aerodynamics, but they're about as long as the Porsche mirrors:
I think the Nissan GTR also supposedly had aerodynamically tuned mirrors:
Also short and stubby!
Finally, the McLaren P1, a car that supposedly had every body detail carefully designed:
Looks a lot like the Porsche mirror!