the 103 / 104 should last at least 20k with no problem.
103's fouling? mines stock other than plugs and steeda sri (no tune) and I run minimum 91 octane. reason being... my gas milage increased as well as butt dyno power (well actual dyno was 114.5whp). 89 ethanol was giving me just shy of 25mpg, 87 was about 26.5mpg and premium shot to 29-35mpg (averages 31).
only about 6k on the plugs, but no sign of wear or stress. and they've seen road trips, auto x, and drag racing in that time.