Taking the rear diffuser to get sprayed gloss black 
I take it you can drive the car without the diffuser attached if needed
?
I presume it's fitted in the same way as the boggo models rear trim. There are a couple of screw the fix to the underside of car somewhere near the center. Without these, the bumper is only held on with the screws in the wheels arches and the clips to the sides of the quarter panels. At speed, the air passing under the car could cause a fair bit of flapage and in turn, compromise the remaining fixings.
It will most probably be ok, but some strategically placed zip ties might be an idea.