I'd be willing to bet that you're still running rich, but putting in new plugs has merely improved it. I don't remember much about your setup from that night at QSL, but I want to say that you have a BOV that vents externally. If that's the case, I'm willing to bet that it's not adjusted correctly (tight enough), and that you're leaking under cruising condtions.

By the way, this is what S13newB was getting at, but I think he got blow of valve and wastegate confused.