I dunno, as I can see in your photo with the spoiler on, there is a bit of warping going on. Could this be due to the unprotected (by paint) plastic being eaten away UV light from the sun? As I scan through some of my favourite pics of E30's, only one has that spoiler on the back... all the rest are nice and smoothe (execpt the M3's of course).
Here are some of my favourite... Euro plastic bumpers, no spoiler:
Yeah, so if you're going to keep it on, paint it to protect it.