I'm talking out of my ass. I'm big into (mostly sim) racing myself, but I have no formal training or experience. You probably know way more about it than me!

If you're a racing nerd then you know how strong the suckage can be. My car uses premium fuel and I get about 7L/100km on the highway. That adds up on long trips, so I try to save fuel when I can. I've tried drafting behind transport trucks. Even at only 90 kmph, I was able to get that number down to 5L/100km.

Electric vehicles have a lot of design features to cut down on aerodynamic and mechanical drag. Special hub caps, no grilles, low drag tires, etc. for the purpose of helping their main problem and selling point: the vehicle's range on a single charge. I assumed the flush door handles were just another design feature for reducing aerodynamic drag, where every little bit counts.

Again, this is all out of my ass. I am well aware that aerodynamics are far far more complex than "smooth = better", and that most cars are probably already designed so the door handles aren't a problem. Maybe the door handles make no difference and having them flush is just optics for Tesla.