I removed my stock gas tank, and found an unused hose that goes from in front of the carb, like on top of the mount, up along the frame to a spot near the steering head then has a plug installed? What the hell is this?
It does a couple of things. Reduces intake noise and in theory it stores air between intake cycles. It's a good thing.
I can vouch that it does what it's designed to do in terms of noise reduction. Late model bikes run quieter and with less mechanical noise than older models, and this is the only mechanical difference between them other than the cam chain tensioner. Any contribution it makes to performance would require dyno runs.
It's definitely a potential intake leak.
You could run an early intake to eliminate it entirely, but since it actually does something I'd leave it rather than cap it off, if only because vacuum caps tend to lose their seal and degrade quicker than decent hose.