Thanks fietsbob.
I hadn't seen the threadless steerer extension before.
I'd seen a threadless stem riser, which clamped over the steerer and had an area for the stem to clamp above, and I'd seen threaded to threadless adaptors (which were basically the same design as what you posted), but I hadn't seen anything advertised as a steerer extension.

I would imagine that, as with a threaded stem or threaded to threadless adapter, this would be slightly less rigid/solid than a standard threadless setup (but not to an unsafe degree). Is there a perceptible difference in feel?