Great thread and a "hot" topic! Backpressure helps reduce sound and too much backpressure also reduces horsepower. Backpressure is usually the enemy and that's why 8-cylendar race cars use balance tubes. However, if the exhaust pipes are too big then you can also lose power because the exhaust gasses won't flow as smoothly and continuously. This could easily cost you 4 horses. The proper system is tuned in such a way that it flows smoothly with minimal restriction, gets the stream of gasses moving and then it actually pulls that last bit of exhaust gas out of the chamber so you can burn more gas. A really big pipe just won't do that. I agree with Joe that a stock M6 exhaust probably has the correct backpressure and flow characteristics for the stock engine, but I don't buy the idea that backpressure is beneficial by itself.
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