I am sure that is correct when you are talking about a fixed size device. But in this case we are also significantly reducing the size of the mirror.
Here. this should help:
I am already fabbing up the spare of the $4 mirror to use on the left size, as I think this will be both simpler than the motorcycle mirror and also have less drag. The motorcycle mirror is about 1/3 the frontal area of the stock mirror, but these things are just tiny.
The drivers side one will get about half of the horizontal distance as flat mirror, and the other as domed blind-spot mirror.
As to mounting these vertically. Yes, it could be done, but not on my car as not enough was exposed when located where it would have needed to be to not be hit by the door. Also, the mirror is domed for horizontal mounting, and is less than useful when mounted vertically. Though as noted it would be simple enough to remove the existing mirror and replace with something else, I will be doing this on the drivers side.