the mass has already added fuel for the air that you are dumping out. so it runs richer while you are coming down and then it dies. the best way to overcome that issue is speed density. (maybe gm maf I've never used it so i don't know) I would route it back in if i were you the car runs and shifts so much better when you do. If it weren't because I know I'm going to go AEM then I would stick with a type R and route it back in.

What bov are you using anyway? FYI the stock plastic one is pointless if you don't route it back in.