I doubt your plugs are causing pinging, or that colder plugs will fix the problem.
Those are hypereuctectic pistons, right? One of the claims for this alloy is that it runs hotter [ needs larger ring gaps ]; this is supposed to make more HP because more heat is retained in the chamber & this keeps gas pressure high; more 'push' on the piston. But a hotter piston top is also more likely to ignite the mixture before the spark does [ detonation ]. They don't tell you that when you buy the pistons....
My advice is to try & keep the incoming air to the engine as cool as possible:
- cold air induction
- improve cooling system
- block off &/or reduce size of exh xover in the intake
- water injection
- all of the above
