With how much the mirror could end up costing you on top of labor, it's probably better to just use the factory convex mirrors and get used to seeing objects closer than they are.
For years I drove semi trucks and trucks of all sizes, so I am very comfortable with mirrors. I never look over my shoulder when backing up, I don't care if I can see out the rear window or not, paint them black for all I care. I also much prefer to back up a trailer with the mirrors. Having said that, good mirrors are a must and some car mirrors leave a lot to be desired.
On any big truck, you will find they use both flat and convex mirrors. Both are very useful. The flat mirror is for your precision backing up and parking operations and the convex is for changing lanes in traffic and seeing the world behind you. Flat mirrors are terrible for changing lanes because they are nothing but blind spots. They really don't show you the whole picture. Many cars I've driven, the right mirror isn't big enough, or convex enough to show the whole picture, so I've supplemented them with smaller round stick on mirrors that are even more convex.
Car companies give you right side convex mirrors because they are most concerned with you safely changing lanes. They figure you will look over your shoulder to back up and now we have back up cameras, bumper beepers and even better, the bird's eye view. However, backing up accurately with convex mirrors is possible too, you just need to get used to them.
I am intrigued with the European/German aspheric mirrors. I imagine them to be flat mostly up close to the car and convex out on far right end. Probably about 70% flat and 30% convex. I might really like them, but I doubt I would go to the expense and trouble to covert to them. Just not worth it too me. I would like to see them if anybody gets their hands on an Ampera e anytime soon.