Where are you getting this 15-17psi of back pressure in the exhaust number? I've never measured it on a stock SC, but that number has to be wrong. Ever drive a car with clogged cats? That sputtering and backfiring and complete loss of power is caused by about 4psi, so with 17psi the car would be a complete dog!
And since I'm responding to this thread, I'll throw my input in. Backpressure in the exhaust system is absolutely horrible under all circumstances in all internal combustion engines, whether gasoline or diesel, supercharged, turbocharged, or naturally aspirated. If your system is creating even half a psi of backpressure at any point in the rpm range, your car would make more horsepower at that same rpm with a freer flowing exhaust system. I can't even tell you how many times I have gone through this in multiple posts in multiple forums, but this backpressure myth still runs rampant on the internet, and it is a pet peeve of mine. Any backpressure is always a bad thing. People frequently confuse backpressure with velocity, but as 90sc35thann pointed out, the laws of physics tell us that it is the exact opposite. In a perfect setup, the perfect exhaust system would be under negative pressure, or vacuum at the rpm where the motor makes peak power. The vacuum will not only help suck the exhaust gases out, but also helps suck the intake charge into the cylinder during the cam overlap, which creates a greater pressure differential between the intake manifold and the cylinder, which fills it with more air and fuel. In case you were wondering, another device that creates a greater pressure differential between the intake manifold and the cylinder is...wait for it...a SUPERCHARGER! So a properly sized exhaust system will have a slight supercharging effect on the motor. The vacuum in the exhaust that creates this scavenging supercharging effect is created by the velocity of the exhaust gas heading towards the tail-pipe. The greater velocity, the less pressure or the more vacuum the engine sees at the exhaust valve, and the more power it makes. At lower rpms there is less exhaust gas to expel, so you don't have much velocity. At higher rpms with a smaller pipe, the exhaust can't flow fast enough and it backs up creating backpressure and eliminating scavenging. With a smaller exhaust system, scavenging occurs at lower rpms, but then immediately after that rpm it starts to become a restriction which creates back-pressure, which as I have said already is bad for power, so you would sacrifice a lot of power on the top end for a very small increase in power on the bottom end. It simply is not worth it. As you go larger with the exhaust system, you need more exhaust gas to fill the pipe so you reduce velocity at low rpms, which raises the point at which this peak vacuum occurs, and if you have sized the system correctly, your scavenging will peak at the same rpm as you make peak horsepower. So in effect the only way of saying a system is too large is if it doesn't see peak velocity until after the max rpm the engine will ever see. In that case, you would lose out on this scavenging effect, but considering how minimal the effect is, and how drastic the effect of a system that is too small, you are always better off erring on the side of too large, from a performance standpoint. In the real world, there are other concerns such as packaging, sound, drone, price, etc. and if you go with a smaller pipe because you want it to be quieter and the slight loss in power is worth it to you, then go for it, but if you are looking at it from a performance standpoint, better to go a little too big than a little too small. Oh, and just to reiterate, BACKPRESSURE IS ALWAYS BAD!
