Which down pipe should I get?

[Ragefear]

False. Exhaust velocity is what gives you the scavenging effect, giving you a lot more low end. Too big and you may gain on the high end, but you end up with no low end to get you there. The exhaust system needs to be balanced, which is why saying that every car should run a 3" DP is just ignorant.

thank you! Someone who understands and doesn't insist on just throwing the biggest exhaust available at the car.

An exhaust that doesn't scavenge is a motor that has to work harder. Depending on the type of boost and how much, it still can be a factor.