Misfires are caused by a lot more than gaps opening up. If you have a good ignition that is less of an issue than other things. Too cold of a plug can foul even with proper tune. Can glaze by not burning off deposits. That will cause a misfire. Many other things besides gaps.
I had misfiring over the winter.. Put the new plugs in there and it came back to life. But I have about 1300 miles on these with various power levels of street pulls as I tweek the tune and it's starting to break up again.
My wires might need replacing tho because I don't get the nice clip feel when I push them over the ends of the plugs.
Sounds like you have a much higher estimate of plugs than i do. Especially while tuneing ive always heard change them often. As soon as you think the tune is good, new set. On motorcycles i change my iridiums every 10k amd thats stock engine, ignition, exhaist intake and a tune. And surely mileage and power go up.