Fabspeed does make some good stuff. The real issue of debate here, as far as I know, is that differing years of Boxsters have 02 sensors after the 2nd cats and some dont. If your car has Lambda sensors after the 2nd cats, and you remove them, yes the ECU is going to be pissed off. It will think that the mixture is way off and try to either compensate or simply run in limp mode. There is also a potential issue when running high flow cats, as you have still changed the flow rates of the exhaust system, as well as the fact that high flow cats are good enough to pass emission testing, are usually not as clean as factory cats, and may still piss off the ECU if there are Lambda sensors after the high flow cats.
I am also thinking about doing the Fabspeed high flow catted headers and cat bypass pipes WITH THEIR ECU, as hypthetically they have mapped it properly and it should run great. Reality is anyone's guess though. At the end of the day, the ideal solution is a custom map for your specific car, with your specific mods, and in your specific location (elevation, temps, humidity, etc).
Patrick
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