Air Power Systems Pty Ltd has this to say about their blow off valve:
APS BOV development:
We are often asked why we developed our own blow off valve when there are so many on the market. Quite frankly APS had not found a blow off valve for the WRX/STI that offered the level of performance required.
With a large turbocharger, releasing pressure from the turbine compressor wheel when the throttle is snapped shut is critical not only for turbocharger response, but for turbocharger longevity. A huge volume of air must be passed through the blow off valve as soon as the throttle is closed.
Most blow off valves have limitations in their design that inhibit the level of performance we demand. Piston/bore diameter and venting ports are often too small and surprisingly, most will not fully expose the exhaust ports when fully opened.
Ultimately, the need for a truly high performance blow off valve specifically designed for the WRX/STI needed to be satisfied.