The IK22 are the same heat range as stock, you shouldn't experience any difference, except that when new, things will be smoother. However that will be the case with any new plug you put in (assuming its the proper heat range for your application). The only advantage of IK22 over stock would be reduced center electrode resistance. The Denso is only 0.40mm, the smaller the better because it reduces quenching and reduces the voltage required to create a spark between the electrode and ground.
Denso is quiet pround of their U-groove design ground claiming it reduces quenching even further. NGK Iridium doesn't have that feature, nor does the stock. I would say the Denso IK22 is a good stock replacement. The IK24's are probably borderline for NA application and somehow my feeling is they were specifically developed for FI applications given their particularly small gap.
If Denso and NGK intended the IK24 and BKR8EIX for NA applications then why wouldn't they pre-gap them larger. If the iridium biggest selling point is the improved spark and elevated melting point, hence they can generate more heat. Shouldn't the gap be larger then to take advantage of that?
I am not sure my logic is correct though because event the IK22 is pre-gapped to 0.32mm. Could the U-groove be a factor when selecting such a small gap on the Denso?