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If anyone is still confused about exhaust sizing, and the difference between NA and FI motors. Here is a good write up expaining why the theory of "too big of exhaust will hurt power" is wrong.
VE= volumetric efficiency
A smartly designed normally aspirated exhaust has a mechanism to convert exhaust gas energy into negative pressure. This is a collector. If the collector and primary tubes are designed properly (and many are not), they will actually enable each exhaust gas pulse to gently 'suck' on the other primary tubes as it passes through the collector and on out into the exhaust pipe. The greater the 'suck' action, the greater the positive effect on VE and the reduction in pumping losses. If the exhaust is enlarged enough to reduce exhaust gas velocity through the collector, the exhaust pulses do not have enough energy to make the collector work, and our power isn't there. This is often misinterpreted as not having enough 'backpressure', which is incorrect. Of course, if the exhaust is too small, this increases pressure in the system, which begins to cut into our power as well. Anything more than ~2 psi in a normally aspirated exhaust system is going to reduce power, so sizing normally aspirated systems is like designing/sizing a musical instrument to sound a specific musical note when we blow into it.
A turbo exhaust has no such mechanism to create something positive from exhaust energy. Once that exhaust pulse spins its way out of the turbine, it can do nothing but get in the way. If pressure is created on the backside of the turbine, it just reduces turbine efficiency and VE as well.
With a turbo car, a larger exhaust will make the most difference at peak power, where the engine is consuming the greatest rate of mass air flow. IF we find that a larger exhaust somehow reduced power, it's because it increased VE and leaned our fuel mixture enough to make a difference. If we re-tune, we will get our power back, plus some.
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