I have used quite a few aftermarket bumper covers before. Typically on higher quality aftermarket covers I think it is ok to clean good, sand/scuff the primer, clean again, seal with epoxy, and paint. However, I will say that I had one bumper cover that was primed that I used these same steps and the paint peeled off on sheets. I literally pushed the bumper into the plastic bumper stay and my hand rubbed the paint off down to the primer. I have never seen that before. Luckily the company replaced the bumper and I was able to fix the issue before the customer even picked up the car.
I do not think I would use promoter over primer. I have always been told that promoter over primer would decrease adhesion, but I do not have any proof of that. Someone might can chime in and correct me if I am wrong.
I guess the full proof way to ensure a perfect job on an aftermarket cover is to remove the primer and start from scratch with a raw platic cover?