A smoke test isn’t a valid means of testing a blow-off valve for leaks – it doesn’t replicate on-boost conditions. The problem with a smoke test is that the plumb back port is pressurised – this never happens when you’re driving the car. On a Deceptor Pro this will cause smoke to escape from the trumpet. However, this doesn’t mean that the valve leaks. Also, the smoke test does not pressurise the manifold, which the BOV relies on to remain shut under boost. As stated above, it’s pretty difficult for a BOV to develop a serious air leak of the sort that would noticeably affect the way the car drives.
IMPORTANT! Note that almost every turbo car has a boost controller. The way a boost controller works is by leaking air, but the leak is nowhere near large enough to drop the boost pressure. Consider this if you think a pin-hole-sized leak is coming from your BOV and is causing your 4psi boost drop.