I installed a Cup Splitter on my car just yesterday. Since the guy from the dealer was not completly sure what is needed to install it he called Porsche in Weissach:

They recommend plugs (one part snaps in, then there's the plug to fix it), but no tape.

I tried it that way but there was a small gap between bumper and splitter, so I removed it again and put some 3M tape between. Now it looks much better....