I have a couple thoughts:
1. a SMALL vacuum leak should not cause an issue near WOT
2. Two of my cars have spark knock, and have had it since new. Both under load, one quite significant - enough that I looked into an aftermarket tune solely to get rid of it. At 130K and 160K I have come to accept that it is normal. If a headgasket ever blows, I will probably attribute it to this.
