I put no faith into gimmick plugs.
Comparing new double plats to new Iridiums to new coppers I doubt anyone would ever be able to measure a difference in the real world.
Most people install fancy smancy new $12 each plugs and say, "Wow, my mileage went up 3 mpg!" Of course, they're comparing new plugs to ones that 100k miles old and completely worn out.
I use cheap coppers in my aspire because it takes 4 seconds to change them. I use double plats in my V8s so I don't have to change them as often because #4 and #8 are a PITA.
FYI, coppers provide all the "performance" of iridiums at a much lesser cost (and life).
NGK Standard Iridium vs. IX Iridium spark plugs?
I'm not aware of the NGK standard Iridium plugs... may be the Double Platinum or the GP Platinum.
I'm currently using the NGK TR55IX's in my 01' Silverado that NAPA Auto Parts had on sale earlier this year without any problems. These are the Iridium version of the NGK Double Platinum's that are OEM from the factory.
A friend of mind is using the NGK TR55 V-power spark plug in his 01' Corvette. He claims to have better low-end with them.
I have used the IXs in a motorcycle (Honda ST1100). Compared to newish standard NGKs, no noticeable chnage in any respect. They may be better in the longer run as they should not deteriorate as quickly, but the bike is relatively easy on standard plugs which laste ages anyway. I put them in as the two back plugs are a pain to get to, so I want to set and forget.