Unless I'm mistaken, an ignition coil is nothing more than a transformer, correct? Just two coils wrapped around an iron core. There's not much you can do to "improve" a transformer.

The only thing you can do is change the turns ratio to provide a higher-than-OEM secondary voltage, like 22KV instead of 20KV. This could potentially wear out the spark plugs quicker, but I suppose it would give you a fuller burn if you were running a little rich.