i'm assuming by "breather" you mean the PCV valve
well.... that depends on your idea of performance
if you want your engine oil to fill up with extremely caustic fumes, and your engine to last about 40-60K miles (maybe) then you should disconnect your PCV valve
a friend at another forum said it better than i could:
The system works like this: Engine vacuum in the intake manifold draws blowby gases from the crankcase through the PCV valve. Fresh air is allowed to enter the crankcase via the hole on the front of valve cover; it is connected to its own little foam air filter in the air breather.
The purpose of the system is two-fold: (1) it is a vehicle emissions control device in that it "re-burns" crankcase blowby gases. (2) It prevents pressure from building up within the crankcase, which would cause excessive oil leakage and premature gasket failure.
Some guys immediately remove this system as "emissions crap," but that's not advisable. Virtually every carburetor made since 1967 is jetted and calibrated to work with this system. Disconnect this system, and your carb will run too rich. Also, if you disable the system, you create positive pressure within the crankcase, which allows oil to get sucked past the rings on the intake stroke, thus increasing the possibility for detonation, which we all know is a bad thing...
positive pressure in the crankcase can induce ring flutter and prevent sealing a vacuum in the crankcase stops that and also reduces windage from the crank and oil whipping
in short, re-install it