The overspray isn't difficult to remove. All you have to do is clay your entire car and all your glass. Its just not very professional to do something that you know is going to spray all over your car. Spray paint can travel a really long way. I sprayed something in my garage last week and I could see the overspray particles in the light beam coming in through the window over 20 feet away almost immediately. If you think that it's not going to get on your car when you are spraying 8 inches from your paint then you are crazy.
Its not that the spray can stuff won't work, its just that the non high temp stuff isn't going to do very well. Your brake calipers probably get hotter than your valve cover does. You need to use high temp engine paint if you are going to paint those calipers. If you don't then you'll find out why they make high temp paints the hard way. Your paint will flake right off the brakes within months. I just spray the paint into a jar and brush it on with a foam brush. It works just as well and nearly as fast. You don't have to spend time masking anything off and it still looks great. It was one of the first things I did to my Legacy when I got it. It makes a huge difference to paint the crusty stock calipers on most cars.