Series 1 RST Stage 1 chipped to approx 185-190 BHP

Have bought some HKS Iridium plugs but car now starts to backfire/stutter when my boost gauge hits about 15PSI so have put my Champions back in for the moment.

I have set the gap on the HKS plugs to 0.8 which is on the HKS website but have been told i may need a different gap setting as the car is chipped????

Anyone know what the gap setting should be and if this will even cure the "backfire/stutter" problem?????

Cheers,

Mark