The 421s arent known to be time bombs like the 535s. Their dome still leans alot to be desired.
Dome design is that important you say? Why yes it is. Improper domes can range from a blubbering impossible to tune mess to something that is a grenade at best. Theres domes that "work" AKA off the shelf stuff designed to work for a broad range of setups (like an adjustable wrench) then theres custom domes which are talored to you engine, riding and fuel. (The correct tool for the job at hand)
The bPr8es plugs have a projected electrode which run a little hohotter. Keep an eye on it.
The carbs use plastic idle screws to prevent chipping the chrome plated slides.