Metz 58 AF-1

Just in case you have missunderstood Matt, I think it should allow speeds above 1/180th with the ISO forced to anything, not just 1600.

Basically, the camera doesn't like doing flash above 1/180th because even with HSS it still looses guide number by doing this. Thus, if you give the camera any degrees of freedom at all it will try to keep the shutter speed bellow 1/180th of a second. For example

you set Av, F8 and Auto ISO. The camera determines 1/180th and ISO 200 for this shot. You then change to F5.6, the camera has two options. It can push the shutter to 1/360 or drop the ISO to 100. The former requires the camera to engage HSS and loose half its flash power, so it will judge the latter a better option and drop the ISO rather than increasing shutter speed.

The K10D has such easy ISO adjustment that I keep it on manual all the time. I have Av, Tv and Sv set to use the front dial for ISO. I find this way I am much less likely to get home and realise it took that great sunset on ISO 1600 (or at least if I do it's my fault!)

I didn't misunderstand

You are quite right - the camera tries its level best to keep below 1/180, because it has no real idea of the flash amount required Using HS mode cuts the guide number in half, as you correctly stated, but the amount of light you actually need is not just based on the ambient exposure - it's based on the scene too (and the camera can't possibly know that).

That's why you'll find George and I recommending fully manual for any controlled situations

Matt