For 15 PSI, you should probably go at least 1 heat range cooler than stock, and you'll just have to gap them to find what's good for that plug. The plugs I use are 2 heat ranges colder and I use .035, and I've boosted a little over 18 PSI with no problems, all the way to redline (also BPU). Try some of the recommended plugs on MKIV.com.