i looked at that website and was quite interested in all that they offer... but with respect to just a 90* release valve on a stock exhaust, i think you should probably see lower boost pressures suddenly as the valve opens, which would cause the valve to close back up again and this would cause it to fluctuate again and again. With a turbo set up, once the turbo is spooling, you want zero backpressure in the exhaust from the turbo back, because that will in fact increase boost moreso than decrease it because you still have the turbo to give back pressure in the exhaust between it and the engine, and the lower pressure on the outlet side of the turbo would also help it spin higher rpm's. As far as it affecting the tune, i really dont know, but i would believe it if someone told me it did, maybe if you had one of these installed after the resonator so that there would still be a relatively stable amount of backpressure in the system with the valve open, this might work. What i would like to see is one of these things being turned around and offered for NA applications operating on vacuum instead of boost, when max vacuum is hit in the intake, the exhaust valve would open giving you basically open headers when really into the throttle, but the question would be, is there enough vacuum to power one of these, what about idle vacuum levels and if more than one valve used as would probably be ideal for a V engine set up, would there be enough vacuum for both? Interesting product though none the less.