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Yeah, I used "wastegate" generically (and probably incorrectly). Really it would be a bypass or blow off valve that is boost pressure actuated, not vacuum actuated. However, depending on whether it is a bypass or blow off valve would affect placement.
A blow off would be best, but may have tuning and drivability problems. It should be placed before an intercooler.
A bypass is what the non-M would need to use, since the supercharger tunes still utilize the MAF sensor. The bypass would be best placed post-intercooler. The air compressed by the supercharger is heated by both the natural heat of compression, and the inefficiency of the supercharger. You want to limit the hot air being recirculated back into the intake, and cool it first if you can. Basically if you convert the temperature from deg F to deg R (or from deg C to deg K), you can figure out how hot the discharge air would be under an ideal, "adiabatic" compression (where the supercharger is 100% efficient and adds no heat of its own, only the natural heat of compression). Then plug in the supercharger's efficiency (pulled from the compressor map) at a particular pressure ratio (y-axis) and volumetric flow (x-axis), and you can find the actual discharge temperature. The actual discharge temperature will be higher than the ideal temperature.
What this means is when the air is recirculated back into the intake and allowed to expand, it will be hotter than ambient. Every time it passes through the compressor and back through the bypass, it gets hotter. After a few cycles it can get very hot. So, this is one reason why there is a point of diminishing returns with this setup. If you use say the 18 psi pulley with a bypass because it initially appears to offer better performance, you will be heating up the air and dumping more than half of it back into the intake to be reheated again. It will add even more heat to the system than I have shown on the graphs. Plus there is more power required to spin the supercharger, and running it at a higher speed will shorten its life.
One last thing to note - too much air flowing back into the intake can affect MAF sensor readings, and it may start "counting the same air twice".
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