In my own (the Cobra), run NGK copper plugs with excellent results. In the past, after reading articles and posts about how more expensive plug A, B, C or even D out perform the cheaper ones, have tried a few. Same car, same tune, same driver, only one other equaled performance of the coppers while rest of them came with hair attached causing this that or another running issue.
Now somewhere between $1,000 and $1500 into spark plugs for the 3V. Absolute best of the bunch, plain copper plugs from Accel. While Brisk plugs are touted as the end all for a 3V, we've experienced the exact opposite seeing south of 100km of run time (or less) before dropping a cyl or cyls due to fouling. Experimented with their 1 plus 2 colder heat range versions with similar results. Ford Racing plugs, dismal results there as well. Only moved to the Accel out of desperation, did not expect things to work out so well but thankful it did. While results with the 3v are not typical of most engines, moving to more expensive plugs in the Cobra showed more expensive is not always the best. Only downside to running a very tight gap, does hurt idle quality some but it is what it is.
I've seen a good number of posts where Cobra guys report performance fall off with the coppers in low miles. I've seen the opposite only replacing every couple of seasons as a matter of good maint, never once a fouled or dead plug. Seeing as they are priced extremely right, just doesn't matter.