high flow exhaust = 3-5% mpg gain?

[Tesla]

Don't know if anyone has read this, but it gives a lot of information in an easy to understand way about exhaust systems, it is about Harley's but it is applicable to all exhaust systems. Main point is you can't just open a big hole at the engine and expect improved performance, there is a lot more science to exhaust systems than there is to intake systems.
RB Racing LSR 2-1 Exhaust Technology