Under some conditions, some aftermarket components DO increase power - and some do under any conditions. IF the OEM plug does not always light the fuel effectively, or if it causes/allows minor detonation, and the aftermarket plug DOES fire the fuel effectively every time, and or prevents low grade detonation, THEN the engine WOULD produce more power and better fuel efficiency with the aftermarket plug.
Fine wire plugs HAVE been proven to fire more consistently, with lower ionizing voltage, than large electrode plugs.
This has been known for a long time. Noble metal fine wire plugs have also been proven to run with a lower tip temperature and retain less heat than a large electrode plug. This has also been known for a long time.
However, getting a noble metal fine wire plug to stand up at a reasonable cost has not been easily accomplished. The fact that the iridiums are only about double the cost of standard plugs and they are lasting as long as or longer than standard platinum plugs means they are finally getting there.
I would not presume that putting Iridium plugs in absolutely any vehicle would provide horsepower and economy gains - but I would be the last to say a 2 to 5% improvement in either or both was impossible, or even unlikely.
Now there are a lot of aftermarket "gee whiz" plugs etc out there that can NOT POSSIBLY provide an increase in either power or economy over a properly gapped and unworn OEM plug. or other properly installed and adjusted OEM part.