Here's something constructive for you from someone who's actually done the modification.
I've been running this mod for a few years on my .308, had it done by a gunsmith, and it's become one of the most valuable things on my hunting rifles ... but only because of the specific way I hunt.
I chase blacktail deer in thick timber on Vancouver Island (similar to the kind of bush hunting, the guys referencing in New Zealand), where the majority of shots are under 100 yards and encounters last seconds. The deer appears, the deer disappears. In that context, any noise during the transition from carrying to shooting can end the stalk. The half-cock position lets me carry with a round chambered and the firing pin physically disengaged - mechanically safer than a standard safe-carry with a cocked firing pin - and go into battery with a silent downward push of the bolt rather than a noisy cycle.
To the replies here: if you're hunting from a stand, walking open country, or anywhere a conventional bolt cycle gives you enough time, you genuinely don't need this and I wouldn't bother. It solves a very specific problem for a very specific style of hunting.
On shearing the bolt handle: Tikka bolts are hardened steel, and if a drop were violent enough to shear a bolt handle at that point, the rifle would have bigger problems regardless of whether the modification had been done. It's the kind of theoretical concern that sounds serious until you think about the actual forces and materials involved.
On "just use the safety" — agreed, and I do. The half-cock position uses the safety to hold the bolt. The firing pin isn't cocked. It's a more mechanically inert state than standard safe-carry, not a less safe one.
Not for everyone. Very much for the hunting I do.