I changed from the stock NGK to NGK Laser irridium (NGK PART No. ifr 6t-11). I subsequently had the Pajero dynotuned again and now have 145 HP on the rear wheels. The prevous best was around 138 HP on the same Dyno. Altitude is 5000 Feet above sea level. Vehicle is a 5 door 3.5 DOHC Pajero with 140 on the clock. There are other modifications but in answer to your question the irridium plugs were expensive but worth it. When changing your plugs use some carburettor cleaner and old toothbrush to clean the intake
plenum and throttle body Air Flap. I got tired of cleaning this all the time and eventually blocked the pipe on the intake side and installed a K&N breather on the motor side. No more junk being sucked into the plenum and runs exactly the same as before.