That will look factory-ish from most other cars, which is what I think you were going for. Nice workmanship!
When cutting thin, brittle plastics with rotary tools, take some painters mask tape and make your cut line on it. Same way you cut a laminate countertop. It will help keep the pieces together and give you a cleaner line.
I would think that a GB from behind would just create giant stagnation areas in every crevice, and be detrimental.
