As long as the base color match, the only issue you have is with the clear coat. The base color is not gloss, the top clear coat is what makes the car look shinny. A lot of body shops don't use fine enough polish after wet sand / buffing. Your best bet is to check micro scratches and orange peel texture on the top clear coat. You'll need to look at the surface at an angle under certain lighting condition. Flex agent is added when painting plastic to give the paint additional flexibility. Plastic bumper is extremely tricky because high speed buffing can brun the paint easily. I think all you need to do is to have the bumper polished with a foam pad using a dual action machine, it's a very time consuming process.