well, I saw the same ebay vendor as you and, while he only has one valve left, he has managed to sell 6 of the 5 units he has ... but yeah, if someone can find a better vendor.
otherwise, I was thinking of cutting-out the secondary cat on each side and putting cutouts in their place using easyseal exhaust clamps. attach the valve to the cutout so that muffler is bypassed when valve open. simple and easy to put the cat back if required.
regarding control, instead of using a switch, use the vacuum switch that opens the resonance flap on your intake (if you by an x51 power kit for your car it doesn't have the resonance flapper and the Porsche install instructions actually tell you to repurpose the rpm-activated vacuum switch to control the pse). so, you can use the same switch and split the vacuum line, or split the rpm signal and use multiple switches. the benefit of the latter is that you can turn off the pse by killing the signal to the switch. either way you should think about adding a second vacuum tank. using this method you get a pse that opens over 4000 rpm when hard on the throttle, but keeps back-pressure at lower rpm and keeps the muffler in the loop when driving sedately.
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