Lets start from the beginning. The difference between the CR9EHIX-9 and the CR9EIA-9 is that the CR9EHIX-9 is not threaded all the way to the base of the plug. The last 1/4" or so is not threaded. This plug is used in lots of Hondas and won't work in the ZX-10R. That said, the CR9E, CR9EK, CR9EIA-9 and the CR9EKPA and the CR9EIX are all pretty much interchangable. They all have the same reach and the same heat range.
Iridium plugs are used because they will last longer between changes than conventional plugs.
It has been found that dual electrode plugs in some applications will produce a slight horsepower gain for reasons unknown.
I will probably stick in a set of the CR9EKs when I take a look at the valves next week and sync the throttles bodies because I have a few sets laying around. I'll let y'all know if my motor blows. When I order parts next I will probably try a set of the CR9EKPA which is the dual electrode version of the O.E.M. plug.