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As most people have touched on, it depends how the performance mod works. Any performance mod which works by reducung the load on the motor will help mpg eg electric fans, underdrive pulleys.
Tuning Mods which make the most of what you have, such as ignition advance curves, mixtures can definately help provided the guy doing the tuning knows thats what you're after.
Rebuild mods which help performace will certainly help mpg, such as increasing compression, balancing and lightening reciprocating and revolving parts, some smoothing of the head ports (although some turbulence in the intake is beneficial to keep the fuel mixed in)
As far as gas flows through the engine, it get s bit more complicated. Smooth airflow will help mpg, the biggest challend is getting the volume of tract correctly matched to where you want the engine to perform the best. Most of us want the best at normal cruising rpms.
In the intake and exhaust, the inertia of the gasses help the engine, particularly the exhaust side. asside form the sonic scavenging effects in tuned length extractors, there is also the kinetic energy built up in the moving exhaust which want to keep moving after the valve closes, this creates a low pressure ready for the next time the valve opens. If your exhaust is too large, the gasses won't have enough velocity and inertia to use this effect.
But really to get down to this stage, you'd need custom cam grinds to make the most of it all aswel.
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