There are 2 butterfly valves or flaps in the stock manifold. They stay closed at low RPM to keep a nice torque curve, and open @4100 I believe to allow more air to pass trough and increase top end HP. I deleted mine in an effort to rid the HP/TQ dip when they open. While that was successful, I lost some low end (sub 4K). If I can find them I will include them, but not sure I'd recommend putting them back in case the screws got loose. But that's up to you.
Sorry for the elaborate description, could've been much shorter.
I did not dyno before/after. I'm not sure what the stock dimensions are. I believe it was opened 2-3mm @ the opening and 1mm on the lower runners.
In terms of gains...I ran my buddy with a TL 6MT before he had PCDs and before I had this manifold. I pulled ~1.5 cars or so? I don't remember. Then we ran again after he did the PCDs and after I did the manifold. The results were the same. At that point, I assumed I gained as much as he did from the PCDs. I cannot say that is true. I'd imagine with the manifold alone, there would be a 5whp gain. With the throttle body bored, I'd imagine 7-8whp total. I'm basing this off pure speculation considering my gains from the 3.7 manifold/TB setup. Although my car has shown CEL P0174, which was stored at the time of dyno. So who knows, I could be losing power. Not really sure. Doubt it though considering I'm @ 270whp lol.