I used Betalink on my diffuser. Bought that from a dealership. The tape that came with it didn't hold very well, so decided to go stronger. Be prepared, the betalink will get all over the place. Wear latex gloves, and be ready to use the solution.
As for the CF spoiler, I installed mine with 3M double sided tape. You don't need the betalink. If you track a lot, it may or may not come off, but I doubt it. The OEM lip was installed with tape too if you notice that when you remove it. I also didn't use a heat gun. I used 3M adhesive remover, fishing line, and some elbow grease. It worked like a charm. The heat gun might have made it come off a little easier, but there's a greater chance of damage to your paint if you're not very careful. I decided the risk wasn't worth it and got the spray adhesive remover. No issues here...
'11 BMW E92 ///M3 - ZCP and DCT
'15 Ford F-250 - Lariat, 6.7 Powerstroke Turbo-diesel