ECS Tank

Evaporative Control System (ECS), is this what you're asking about? If it is, then no it will not hurt performance. If you have all the other parts for the ECS then I would keep it because it provides a good tank venting system (and keeps your car original). If you do not have all the other parts to make the ECS system complete, then you can switch to a tank with one vent line and fabricate your own vent. If you do this make sure the vent line makes a loop above the height of your filler pipe and then vents outside the car/trunk.

Above all else, make sure your tank is vented and it's done safely.