It has yet to be proven that any exhaust either hurts or helps horsepower, the main reason to switch is because you like the cool sound... or because of weight.
The Stage 2 Sport Exhaust is a lightweight option, but it requires modifying the diffuser opening and that is SCCA stock class illegal. Otherwise it looks like a good option.
The Quicksilver exhaust is 11 pounds lighter than stock and works very well. We are running it on my car. The downside of the exhaust is the dBA noise at higher rpms. It makes us illegal (93dBA @ 50') here at Qualcomm... so we are designing a custom noise diverter to attach to the tips.
None of this is that big of a deal for autocross, the stock exhaust will work well for 99% of the drivers. But when you win and lose by less than 1/10th of a second, you might start thinking about that 11 pounds of exhaust weight.
