any irregular opening, or irregularly shaped plastic will interfere with the antennas ability to receive optimally. if its a piece of plastic that has a constant smooth form, like a bumper cover you can get away with it, as long as your paint doesnt have metallic flakes in it...
Even more adversely effecting performance, ANY metallic coating, whether its painted on, real or fake, will interfere with microwaves, reflect them back and degrade the performance of the antenna.
i was going to do this install on my 328 and the only suitable location was it poking out through the lower plastic mesh, even with the mesh. On my 993, i would put it behind the plastic mesh knowing i would lose some performance but not modify the mesh, the mesh is irregular shape but its not metallic so its not that bad. The Qi45 and the 9500ci I installed in both my trucks have 'open air' views of the road ahead.
and another i installed on my ranger. i had to raise the antenna bracket to allow it to see through the chrome grill slots. this radar worked great, even though i was concerned about its proximity to the chrome.