Any problems to the engine driving with downpipes + non-res + closed valves?

[pacmansilva]

I apologize for my ignorance, but would like to know the answer to the above question. I currently have akra downpipes and Match Schnell EVC and was thinking about adding a non-resonated exhaust. Since the setup would be very loud, I would have times when I would like to close the valves, and I was wondering if it would have any negative effects on the engine. A guy from a local shop told me that if I were to drive hard (I don't know how hard he was talking about) with downpipes + non-res exhausts and closed valves, it may send pressure all the way back to the engine and cause some damage. I'm not very knowledgable about the mechanics and was wondering if this guy was full of it or any truth to what he said. I could take my downpipes off and just put non-res exhausts, but kinda addicted to the extra power from the downpipes .Any help would be appreciated.