I think Evoboy is refering to an aftermarket type BOV rather than the OEM recirculating valve (Your question was slightly misleading as it referred to a BOV http://www.ltregister.f9.co.uk/graphics/happy1.gif border|EQU| 0 align|EQU| absmiddle >).
At idle a leaking recirc valve won't make any difference to how the engine is running as no air is getting outside of the intake system.
Ref leaking at full boost, the wastegate and amp; ECU will compensate for some leakage anyway. If it's leaking enough for boost drop then there is something seriously wrong.
A way to check for leakage due to a weak spring for example would be comparing the boost setting with the valve fitted correctly and then reversed (When the valve is reversed it is using double pressure to keep it shut). Once you've done this experiment, change the valve back again (even if the car feels quicker) as it causes compressor stall which isn't good.