Apparently I've been running the wrong spark plugs in my car for the last 10k miles. Could I done long-term damage to my engine?

Unbeknownst to me, the SVT cars run a much colder plug than stock Zetec engines even though literally every parts store and website besides the one above show them using the same ones. And I, in my infinite wisdom, stupidly trusted the counter jockeys to actually be informed, which of course they aren't. Hell, no one is: I've spent years on four separate forums and the amount of misinformation about our cars is staggering; on this spark plug issue every forum thread has a different recommendation, and even the most well-respected Focus parts website (screenshotted) shows the right heat range but the wrong part number according to Denso's website. The only way I know for sure that I'm running the wrong plugs is calling a Ford dealership and talking to the SVT associate, who then had to call corporate to get a straight answer because he didn't know either.

I'm not a panicky person but I'm worried about long-term damage: I know spark knock can wreak havoc on cylinder heads, which in my case, could be a car killer. Cosworth only built a few hundred extra heads for these cars (a lot of which were reserved for the Formula Ford teams) so if it goes there's almost zero chance of finding another. The car seems fine: I get good gas mileage, the power comes on clean and strong, there's no misfires or rough idle or anything out of the ordinary besides a bit of sluggishness after a cold start, which I attributed to the crappy PCM tune on the '03's. Should I be concerned or am I just freaking out about nothing?