Plugs can be funny things, obviously, but I've been running the same set of NGK B6HS in my iron head for tens of thousands of miles. (Also on my B31.) The only other plugs I have run on were Bosch forgotten-whats, also OK. (I don't tend to run at full throttle though, I'm a 2/3rds kind of bloke, so maybe I haven't tested them hard enough!.)
No additives in the unleaded - I use the '98' petrol, because when I have used the lower grade '95' stuff (mostly 'E10' now) I have occasionally found float needles get sticky if you leave things for a week or so. Whether this is true cause and effect or just paranoia, I don't know.