The vent guards on my work truck are the stick on 3m deals and they have been through plenty of drive through car wash time with no sign of coming off.
Properly applied these items will not fall off and you should have no concerns. Look at all the vehicles with moldings running down the sides of them, those don't just fall off without a ton of effort. Granted they are a bit smoother and streamline/tight to the body than some of these scoops.
You just have to be sure to prep properly. Be anal and clean up the area before install. Depending on your location, make sure the surfaces are relatively warm and if possible let the car sit in the sun for a few hours after install to assure good adherence. My 1/4 scoops and such routinely see 130-140 on the speedo and they show no sign of falling off.
My rear and side louvers are adhered with 3M. As long as you use a true quality 3M product (with a high rating designed for the application) and prep, you will be fine.
If you have any question if the tape provided with an aftermarket product is a "true 3M" tape - buy the 3M separate.
If you go to an auto parts store you can find these wipes called "prep wipes." I think Dupont makes them but I'm not sure. They are lint free, and pre-moistened with a cleaner solvent. I have used them and they work well and are safe for the clear coat. If you use something you mix up, make sure you use a lint free towel. I would probably say don't use terry cloth or paper towels as they tend to have lint. Maybe try a microfiber cloth?
Anybody know a place to get the good quality 3M tape? One of the brackets of my Roush louvers is loose and I want to get it fixed. The local Pep Boys/Autozone only has a lowing rating 3M "grey" tape that I originally got.