Increased MPG

Jordo!

Generally speaking, power making mods increase the volume of air you take in. That means you need more fuel to mix with it in order to maintain the same AFR (maintained via ECU trimming fuel to target AFR, BTW), which means a lower MPG.

The only exception would be if the mixture (target) were leaned out via tune, which may or may not affect power -- the OEM tune is pretty rich up top ("wasted" to cool the cylinders rather than be burnt), so with no other breather mods involved, you could theoretically pick up a little power and improve MPG that way.

With breather mods and some (safe) leaning of the mixture, you'd probably break about even at best. That said, there are limits to how lean you can go to make best power, so the general rule of more air demands more fuel still applies...

Best way to improve MPG would be to drive in the highest gear possible, and to go easy on the throttle.

Next best way would be to make sure your tires are properly inflated.

Last would be to do some weight reduction.

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