How long is the hose connecting the TB and the IC that you say this cut is in? My intercooler almost rides right on the TB the hose is so short. If the cut is all the way through the hose then... duh!
You should replace that ASAP on the principle of the thing, and also to eliminate that as the source of the problem. It would make sense that the tear only comes open under boost. It would also make sense that the tear would come open when you let off the throttle because there is suddenly a lot of pressure on the TB and the IC as the butterfly closes, that's the pressure the BOV releases. Or in your case your hillbilly BOV (the cut)
On the larger scale, if that cut is letting any air in when the engine is not running at boost (vacuum) then you may very likely be sucking in unfiltered air right into the engine. Not only that but it's likely right under the hood scoop and getting lots of debris flowing past it. Get that replaced right away and don't drive through lots of dust.