a lot of misconceptions here
first off all forced induction vehicles for the last 20 years have had a blow-off/bypass system. it is neccessary to increase compressor life. what happens when you are in boost and slam the throttle plate closed is that you get a backwards pressure spike that slams back against the compressor blades. this is called compressor surge. now the approach most automotive companies employ is the bypass/hooter valve. what this does is when the throttle is closed, or now when less boost is needed they open a valve allowing the compressed charged air to recirculate back into the turbo' s suction pipe / intake duct. this prevents compressor surge, but also allows the compressor to remain spooled. when the throttle is reopened, or more boost pressure is needed, the bypass valve is closed and charge air continues into the throttle body like normal. a blow-off valve for some stupid reason is favored by the tuner crowd, mainly for the noise they make, as it indicates to other cars you have forced induction. the down side is that it wastes the charged air already built up. instead of re-circulating it, they vent it to the atmosphere.
second i believe i read somewhere on gm's tech info that the redline will already come equipped with a 3" exhast, however because it has a close coupled cat, there is no downpipe per se. the cat functions as the downpipe, directly mating to the turbo's exhast output. considering who did the forced induction work (saab) and given their past experince (twenty plus years) and the current implementations on the market from them, the exhast from the factory should be very good indeed.
finally the OBDIII spec has not been finished yet, but MY2007 will require a CAN bus for all Generic Mode OBDII implementations by all manufactors, which could be considered OBD 2.5 but has been refered to, incorrectly, as OBDIII. the main sticking point for OBDIII is that they want a complete telematics system implementation required for all vehicles that would report to federal officials of any emiisions related or reducing effiency failures, who would then send you a smog like notice requireing you to have it fixed.