I would recommend stock gapping if going with a Motorcraft or replacement OEM coil pack. If using the Screamin' Demon pack, open them up to .65, that's what they recommend.
Ford engineers recommend .54 gapping on stock packs for a reason, I doubt you'd see noticeable increases in fuel efficiency anyways.
I went with the Screamin' Demon because it was cheaper than a Motorcraft and I'd read good reviews on them. I have not noticed any increase in power or fuel efficiency, it seems to idle a lot smoother than previous though (in park at 600 RPMS)
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