At least 20 years ago I bought a Bridgestone RB-2. It came from the factory with about a 5mm spacer under the threaded headset lock nut. I've completely rebuilt that bike several times over the years but, since the headtube length and steerer tube length haven't changed, the spacer remains (I just checked).

I don't think I would use a spacer much wider than 5mm on a threaded steertube. It's different from threadless spacer stacks because the steerer tube has threads. It looks to me like the nifty little thread grooves might provide a weak area to concentrate all the stress and snap off. It might be fine but, since there's other ways of adjusting handlebar height, I'd stick with the conventional.