If I get an aftermarket ignition coil with a higher output, run a bigger gap on the spark plugs, I should get more power per my understanding. But what about gas mileage? Would my mileage go down or is the improved power just a result of a cleaner fuel burn? thanks

 

You would think that it would improve mileage, but I don't know. Anyone?

 

Due to the laws of physics, a coil will only put out enough voltage to jump the spark plug gap. The stock coil is all you need.

A bigger spark does not necessarily mean more power.
There could be a delay in when the plug fires in relation to the compression stroke. Would be the same issue with running high octane fuel.