As long as you have the "Extreme" (red backing) tape on there, that thing is never coming off. My front lip is held on primarily by tape and has been through some eyebrow-raising forces, including @DomTheGhost lifting himself off the ground by pulling on it.

 

Try looking for Sikaflex Polyurethane Sealant/Adhesive or the like. I've installed my cousin's Honda Fit spoiler & my old Montero body kit w/ that. It's single component, bonds permanently but remains flexible.

 

The main thing is to make sure you have a very, very clean surface on both items. Use 90% rubbing alcohol to clean both areas PRIOR to attaching the 3M tape.

Not having mechanical fastening is sort of scary but it can work. Also, the panels must fit perfectly with no undue pressure causing them to want to lift from each other.

I would keep an eye on it after install and recheck it every few months to make sure it is not becoming loose as finding out the hard way would be disastrous.

Remember, they glue airplane parts that are exposed to 500MPH speeds so as long as it is done right, it can hold. I believe it is DOW that created a glue that bonds plastics to metals and metals to metals without requiring rivets or soldering.

 

BMW/MINI use Betalink K1 adhesive. That's the 1 made by DOW, I believe. The advantage of sealant adehsive over tape is you can fill the gaps & create a bigger bond surface. PU adhesives like those from Sika work just as well. My old Montero kit came w/ that.