Spark plugs and heat range vs. level of tuning

Hello,

I have a few noobie questions about spark plugs and heat range vs. tuning.

Is there a rule of thumb when to change a heat range? How much you need to tune to go for colder spark plugs?

For example 1.6 GTI runs with NGK BP7ES (or with warmer NGK BP6ES). When to change to Iridium NGK BPR7EIX or even Iridium NGK BPR8EIX?

My 1.6 GTI (EG) runs great from dyno with NGK BP6ES (92kW/162Nm, with original exhaust manifold). Spark plugs looked great after 18 months.
If I go for NGK BP7ES (+dyno) or even a more colder and expensive Iridium BPR8EIX what are the pros and cons? More torque? Too warm engine?
Would NGK BPR7EIX be "cold enough" with extra torque and give good spark even at red line?

Usually tuned engine needs colder spark plugs, yes, but how much you need to tune to go for 8 in (NKG) heat range? Is there rule of thumb?

To remind you, I included here some "equal" heat ranges from different manufacturers:  


I know some of you have tested different heat ranges at dyno…please share your knowledge

Hello and welcome to the club, I'm afraid I am no help on this, it's way over my head lol. Hopefully someone will be able to answer your questions soon though.

Andy.