There was a time, many years ago, when manufacturers routinely installed parts that would have been considered subpar by any aftermarket standard. In those days, power gains were easy to come by. Now, the gap between good original equipment parts and the best aftermarket parts has closed to near nil. The NGK Laser Iridium plug -- used in the Evo VIII and Lotus Elise, among other applications -- is one fine example of that. But the Laser Iridium is also twice the price of comparable offerings like the Denso Iridium. That's a pretty steep premium for something that may or may not be twice as good.
Both the NGK and Denso plugs use platinum-tipped grounds, and both use very fine iridium electrode tips. Iridium is used because of its incredibly high melting point, which allows manufacturers to make the electrode tip smaller. A smaller tip makes for a hotter spark, increasing spark voltage and intensity kind of like a nozzle on a hose increases the water pressure coming out. The white-hot spark ignites the fuel mixture faster, allowing for a quicker burn, more complete combustion, and more accurate timing. Iridium effectively lasts forever in most applications, and the platinum tip adds even more to the durability.
The primary feature of both plugs is the very fine tip, and smaller is better. Both use tapered tips that offer good stability at high temperatures. The NGK's electrode at its tip measures 0.6 mm, which is identical to the standard Denso with a platinum tip. However, the Denso Iridium Power plug, which lacks the platinum coating, has an even finer 0.4 mm wire. That's good for boosting power in the most extreme applications, particularly when used with turbos and superchargers. However, the lack of a platinum ground is going to reduce longevity and increase fouling. The Denso Power would be a good plug for highly tuned weekend racers, but probably not the best choice for cars that see a lot of street miles.
As a whole, considering all the details, the NGK plug is probably the better constructed of the two. It uses a small disc of platinum welded to the ground, instead of a thin coating of electroplated platinum as the Denso uses. The Denso uses a "U-Groove" ground that helps to ensure a more consistent spark, but the NGK's tapered ground and platinum disc do a better job of staying out of the way of the flame front. The NGK also uses a "trivalent" coating on the plug to prevent seizing, making plug changes easier. NGK says the Laser Iridium is the "best OEM spark plug available," and it's difficult to argue the point.
As of 2014, the Denso plugs run around $8.00 a piece, to the NGK's $17.00 plus. That two-to-one difference in price manifests almost directly in sales numbers. That strongly suggests that many people are opting for the cheaper Denso to replace their NGKs after tuneups.
In terms of details and construction, the NGK is the better of the two plugs. If you're looking for one of the absolute best spark plugs money can buy, the NGK won't disappoint. Users report that it holds up better than the Denso in when subjected to high-pressure turbo boost and nitrous -- which is why Mitsubishi specified it for the Evo VIII. So, if you're building a big-power, turbocharged endurance engine, and you have the budget, spend a little extra on the NGK plugs. However, for most buyers, Denso's Iridium should deliver most of the performance and longevity of the an NGK Laser Iridium at half the price.