it really is determined by conditions, for example my uber rich jetted kitted tomos w a puney 15mm overbored sha 74 jet will rip with a 4L
on normal days 65f-75f, when its like 90f it will rip with 6L, when its 32f and below it loves a super cold klg 220 which on the ngk scale is a 10.
i recently rejetted to a 70, and the 4l is an awesome cocoa tan, read the plug it knows, before it was dark chocolate w/ a 74 and would stumble at WOT and cut at idle, being too rich with a 8hs.
almost any weather condition can change which plug runs best, humidity and quality of the gas, oil ratio, timing, altitude and how well your kit sheds heat.
i tried the recomended buz8h and it ran ok but fowled too easy. i have an advantage, cause the harley shops kid ran 2t race bikes in the 70's and has a stockpile of discontinued 2t plugs, hence the ubercool klg's.