On my '05 Wide Glide, I took it from 88 to 95 when it had 7,000 miles on it. 10.5:1 cr CP forged pistons, S&S gear drive 585 cams, NRHS heads, etc. 107 hp/107 tq. The dyno operator suggested I put in a hotter plug, so he used V-Rod plugs; sorry I don't have the plug number in front of me right now. It was still running great at 47K miles. I thought I better change them, so I put new V-Rod plugs in it, and can't tell it runs any better. Plugs still looked decent with 40K on them. I kept them for a road spare.
Prior to the 95 upgrade, I had an occasional check engine light. Code was rear plug misfiring, even tho' you couldn't feel or hear it misfiring. The engine light only came on if I was really pushing it. Normal riding, the light would eventually clear itself. Replaced the plug, problem fixed.
If plugs were getting old, needing replaced, misfiring, etc., wouldn't the diagnostics give you an check engine light?