You have to realize that once an engine reaches a boost level the wastegate will remain open to maintain the boost level. Although it does modulate a little, it will pretty much be open the whole time the engine is under load. As RPM goes up, pressure and flow goes up both in the downpipe and on the wastegate dump. In non linear flow is entering the downpipe via the wastegate it will hurt flow. The real question is how much??? I dont know, but I would rather not have to redo it later if it presented a problem. I may end up fabricating a 5 inch race only downpipe and I can integrate the wastegate to that. Of course at that point noise is a mute point
. I will probably not do that until the engine is tuned and I have some kind of electronic knock sensing.
The small version is what I am basically building. I might check for a damaged new muffer and just cut it up for the packing.