Now that's interesting. I switched to coppers as part of my first "who knows what the previous owner did or didn't do lately" updates. I don't think Motorcraft, that's what I went in thinking I'd get but they didn't have it. Maybe Autolites. I've had fewer issues with coppers than anything on cars that are spark snobs.I know this may sound silly, but where did you buy your plugs? What brand/model?
I changed the starter in our '08 last year and put in new Iridium plugs while I was in there. No CEL or codes, but after the change, we went from averaging around 13.6-13.9 indicated on virtually all tanks of fuel to 11.5-12.5 on all tanks. We've got almost 200k miles on our 4x4. KS roads, but a lot of short trips less than 3 miles.
I hate changing plugs that only have 10k miles or so on them, but I think they may not be genuine plugs or maybe this brand is junk. I did buy via Rock Auto, but they just don't seem quite as good as the old Bosch set.