Ahhh $hit.....your right. I got caught up reading and replying to the other post's and thought he was reffering to his clutch cover because that is something else that gets snorkeled. My bad. good catch.
So as a more relevant reply to the OP post. Adding another snorkle or openings to help the engine breath does add some performance IMO, but it may not be what you think. On my car, when I added 5 small filtered holes to help it breath, all it did was help the throttle response be a little quicker. Its very slight but noticeable. I also have an aftermarket exhaust which also help's the throttle response improve.
I posted pics a while back where I drilled and ran a second snorkel for my air box and honestly guys all it did was bring performance back to how it was before I originally snorkeled it. It mostly compensated for the amount of distance the air now has to travel to get into the box.