maybe i should have been more clear. shim for the nitro % you are using yes. but there are some people who dont and a lot of .21's are shimmed for 25% nitro because iirc thats the max they can run in sanctioned events in europe. so if those people who dont reshim use 30%, like a lot of use do here in the states, they wont be able to use a hotter plug and get the most performace possible out of thier engine.
plugs are just like your timing in a real car. trade your nitro % for boost. your tuning variables are nitro, compression, and timing curve. if you want lots of peaky power lower compression (shim) up the nitro (our version of boost) and run the hottest plug you can get to not burn up or detonate. if you want smoother power. raise compression, go down to 20% nitro and a more medium plug. its still an engine and has to abide by the same rules all engines do. while i may not be a pro nitro engine tuner or modifier. i am very familar with what it takes to make power and create a certain powerband by tuning 1:1 cars and found that it applies to my nitro engines as well through experimentation.