I would advise against hollowing out a stock downpipe. I am not sure but I think I saw a thread on here somewhere where the person did that and they were getting a droning sound or something like that.
I would recommend buying the BMS downpipe because of the price, quality and customer service. I also believe that BMS just released their downpipe fix. There are probably a few threads about that. Just check around
You could also pair the downpipe with the BMS tune and it will remove the light from your dash and clear the codes. Plus those two mod combos work great together. You get more sound out of the stock exhaust and more performance with the tune.
Best of luck!