If selected ON during taxi the system does a self check which opens and closes the WAI valve. If the valve is open for more than 30s on the ground it will bring up an ECAM caution.... no go. If you don't need it for takeoff you MUST switch it off! During the takeoff run the valves remain closed and if the system is left selected ON the valve opens as soon as the aircraft is airborne.
From memory there is about a two tonne penalty on TOW... but I've never used it for takeoff in 12 years.
A4