Not so long time ago my friend shared with me his experience about using spark plugs. The conclusion he made - expensive doesn't mean the best. My buddy Maxim takes part in competitions on tracks and rally-sprints with his Honda Civic EG6 SiR-II with the engine B16A (170hp) and he used two kinds of spark plugs. The first one is Denso Iridium IK20 with mileage 11,000 kilometers most of them were on track with 5500-8100 rpm. In theory 11k kilometers for iridium spark plugs is nothing, but ceramic insulator became black, however electrodes were just well.
Just for the test Max tried usual spark plugs NGK BKR6E-N-11 which are listed in the manual. I'd light to highlight the price - $11 for 4 NGK plugs against $63 for IK20 iridium plugs.
After few testing laps we saw the result - acceleration time from 0 to 60 km/h lowered for 0,2 seconds, acceleration time from 0 to 100 km/h lowered for 0,4 seconds. For those who don't know - it's great result, because in the half a second may be from 4-5 cars!
Another advantage of NGK plugs is that you may replace them every time you change the oil (for those who compete in a sport it happens quite often) because of the low price of the spark plugs.