I'm a new e90 owner. Just picked up a sedan with a sports package and 47K miles. I'm loving it.
Anyway, I owned an e36 M3 and it didn't have the flap, but the same year 328i had the flap. It was on the 328i for sound control and to make the cats warm up a little faster.
I started my e90 cold and noticed for the first 40-50 seconds the flap was closed. I don't know if it closes back up or not. It could just be a cold start function.
I know that there are no performance exhaust systems that have this.
I didn't do either the ball bearing/golf tee or unplug the connector. Unplugging the connector may register a fault because you have opened the circuit. Whether or not it matters is another story.
I went on eBay and did a search for the following after reading the first couple of pages:
PINCH VALVE CLAMP
I bought a couple for ~$3.
I'll put it on the hose inside the panel in the trunk.