ignition shouldnt change plug choice or gap , some say they can run wider gap with the tr6 but that leads to excessive load on the coil trying to spark at high boost ,,result is an early death of the coil pack and then misfires
when it comes to boosted you want a cold enough plug that wont hold heat and become a hot spot and wont be so cold as to foul out
but the trade off is that range may be too cold for normal driving and may foul sooner
the r43ts will run fairly clean normal driving and on alky should be ok to mid 20s boost provide the charge temps are dropping with the alky , above that boost level you're taking a gamble on preignition (the real parts killer) with a glowing plug and should run colder plug
like tuning where you want to start rich and tune leaner to see if there are gains rather than end up lean and torch a motor .. it always best to run colder plug first then work back toward a hotter plug rather than run too hot a plug and loose a motor