As the #2 starts getting prespooled it beings to build boost. If the boost gets high enough before both valves that control #2 open fully, the reed valve serves as a bypass and lets the boost start flowing into the intake path. Basically a pressure relief as #2 comes on line with #1. Sits on top on the passenger side of the engine in the intake path. You can see it sinces it's inside the piping. Only the cover to it can be seen.
Speculation, but I don't think it would hurt performance. I believe this valve is pretty small and any leakage shouldn't really affect the spooling of the turbo. Maybe even make it spool quicker. The transition of prespooling and the vavles opening happens pretty fast.