Revzilla article outlines "a" way to fit steering head bearings...  not necessarily "the only correct" way. 

Preloading tapered steering head bearings with torque on the initial nut is the factory service procedure used by many motorcycle manufacturers which is what was done here.  Reportedly the steering is free and smooth and the bike handles well which is what's important.  Your bike, your choice. 

I thing it’s pretty hard to destroy a tapered roller bearing with a bit of preload, in fact many applications with these such as rear ends/axles define a preload.  But preloading original balls would not be good imo.  As long as races are seated and aligned then all that’s needed - again imo - is to remove all slack, for either.  I have 6 bikes where I repacked steering and zero issues with handling with original balls and see no reason to replace these with tapered rollers.  Maybe the best solution if all else fails is to go back to a good set of races and balls?

Most modern sports bikes have returned to cup and cone type bearings, with the balls caged rather than loose, like the DT Yamahas:

parts.streetbike.co.uk/product/genuine-s...ng-stem-bearing-kit/

I use similar head bearings on TY Yams, steering seems slightly better than taper rollers, they are a lot cheaper, and with a grease nipple fitted to the headstock, should last a good long time.

My steering doesnt have any resistance, so I did the bearing conversion in line with what that RevZilla article suggests, by feel. Yes I loosen the triple clamps when adusting. I just went the extra mile of defining it with a specific torque figure.

The reason I used these tapered bearings was because my old ones were notched and some pitting and the all-balls kit was available fairly cheaply with 2 day shipping..it would have added time and only saved me 20 bucks to get ball bearings.
Yes, they use ball bearings in motoGP, for the ultimate in steering feel. Those bikes also have stabilizers, and much stiffer forks and chassis that wont induce weave until much faster. and I've found ball bearings usually dont last as long...

I thought for a Yamaha Enduro, an offroad oriented bike with crude suspension that might see high loads at the steering head, on offroad tires with flexy forks and no steering feel to speak of anyway, could benefit from the stronger tapered roller design. I am asking a lot of my old DT, on soft Shinko Trials tires, to not wobble over rough roads at 80mph and also stay nimble while not using any stabilizer, but I've found some settings that are working for me. I put a bunch more miles on the bike this week and happy to report it is still behaving. Having enough fun with it that my head gasket let go! I am going to anneal it and keep on riding.....

What tool did you use to torque the castellated steering stem nut?

I could not find any reference to a UK definition for a castellated nut...  in the US, it has a very specific meaning.  In the interest of promoting clear communication:

A castellated nut (aka castle nut, or the similar slotted nut) is a hex nut that has slots that a cotter pin can be inserted through to lock it against rotation... they are commonly used on Yamaha axle nuts and generally take a standard socket or wrench.  I've never seen one on Yamaha steering stem.  (And yes, they are named because they look like the top of a castle turret)

i.pinimg.com/originals/1f/ed/cd/1fedcdf6...cc7b1ee0e2baaf60.jpg

Bearing retainer nuts are round (not hex) and have notches or holes around the circumference in lieu of a hexagonal shape and take a special wrench or socket.  (Or punch and hammer is often used.)   Does not look at all like a castle turret. 

www.witmermotorservice.com/N-12-Bearing-...e/item/N-12?image=03

The meaning of the word castellated isnt specific to the UK.

Some sort of castellated socket, is required to tighten a castellated nut to a specific torque, and just wondered what is commonly used, as its not something I have ever seen myself?

www.vocabulary.com/dictionary/castellated

Slotted locknut....

Slotted lock nut, castellated nut, whatever. What tool is used to torque these down?

You can buy a special socket or use a spanner as shown in Yamaha manuals.  I posted a picture of the nut being torqued back in post #51

P.S. The definition you posted clearly say "like those in a battlement".  No battlement (a castle turret is a type of battlement) I've ever seen has vertical slots around the circumference.  So the steering nut does not meet the "vocabulary definition" of castellated.  Definition of battlement (with example pictures)  www.merriam-webster.com/dictionary/battlement

And it really doesn't matter in the scheme of things.  You can call it a diddly-do for all I care.