You are correct - it shouldn't make a difference. But the only difference I saw was the diameter of the pipe compared between the GHL/ebay one - ebay one had smaller diameter - maybe that restricts the exit of the exhaust from the wastegate just enough to allow it to overboost - after all , I'm only talking about .1bar here.
Like I said, just sharing what I experienced. If you don't experience it, great. But if you are bolting one on for the first time, just pay attention to the boost level - that's all.