I know, old thread, but I wanted to chime in.
Changed out my plugs today at about 49k miles for the denso ik22s. Stock plugs were white (lean).
I'm no pro, but I've done jetting on keihin FCR carbs enough to recognize a lean plug which to me seems out of place on a very healthy and well maintained car with modern computerization. Now, the stuff the OP was saying was something I actually stumbled upon as a subscriber to superstreet magazine. I'll tag the article/copy paste. Those dudes claimed that the stock plugs created a hot spot in the cylinders. Sounds right to me cuz all 4 of my plugs were evenly white. Switched to the densos and yeah, i cant tell *much* difference, but check for yourself, yeah? I'm not sure if its the smaller gap, the geometry, or the cleanliness of the new ones, but its just one set of plugs, won't break the bank. Lean is hot, heat is wear, and this thing aint frickin fast enough to merit a short lifespan. I hope this helps. Not here to argue, i just want to contribute and help fellow lovers of the ge8. Heres the quote:

"The Spark Plug Theory
If anyone else had told me I might pick up a few horsepower with a different set of plugs, I would have probably laughed and moved on. But I've learned not to doubt Oscar Jackson very often, as he has a funny way of delivering on every claim he makes. Since changing the plugs on a GE was a first for him, as well as me, we started pulling parts out of the bay until we found the proper route.

With the new plugs installed, Oscar let the car get hot at idle. That's not a mistake, he let it get really hot. Instead of trying to cheat the dyno by running the car much cooler, or covering the intake manifold with cold towels, he prefers a real world test. He states "I could run the car right away, before it gets really hot and probably pull a better number, but what good is that? In the real world, your car gets hot, REALLY hot. Why not measure something realistic?" After a few pulls, the average number is 104.5 at peak. It might not sound like much, but think about the motor we're dealing with. His theory proves that for the price of a set of spark plugs, you can make as much power as an exhaust or an intake on your Fit. People like Oscar Jackson are what keep our industry moving. No one else is willing to do the tedious research, while he relishes in it. Kraftwerks has these in stock, and they're just a phone call away."
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http://www.superstreetonline.com/how-to/project-car/htup-0910-project-2009-honda-fit/

 

A 5 degree change in air temperature will have more affect on power than a different plug on a NA stock motor..

Been tuning and playing with track cars for 25 years, have put several in record books, so its not like I haven't tried different plugs, the biggest difference is lifespan of the plug. Some really tuned engines need specific adjustments to take advantage of upgrades, sometimes you can tweek tight enough that one plug or the other works a little better. But that also means that if your that tight on your tune that on a long race your probably going to lose something and drop off on power. If your plugs are not indexed when you start ,, its all smoke and mirrors..

BTW there is not a small shop dyno out there that has a margin of error better than 10 horsepower.. TO get down to < 1 HP its places like the Bosche engine lab in Stutgart, or Custom high end engine shops for F1/Nascar etc.. Because the cost goes from 5 digits to 7.

 

So ultimately- with the cost of IK22's being only marginally more expensive that a direct stock replacement, is there ANY downside to 'upgrading' at the time a new set of plugs is warranted? Already in there, might as well do IK22s?