Auto focus on a Sigma 300mm/2.8 with 2x converter

I am about to buy a Sigma 300mm/f2.8 lens. At the shop I tried the auto focus on my Nikon D2HS and it worked fine. Good fast focus when I manually focussed at one end of the settings and then let the camera focus it.

However, I then added my Sigma 2x converter. The lens just refused to focus, either manually or on auto. I was pointing it at the same target with lots of light and good contrast between the target (cluster of telephone aerials mounted on a framework against the sky) and background.
My question is:

The lens seems fine when used with just the camera, so should it still autofocus when I add the 2x converter or is there a problem wikth the lens/converter.

Will the lens work properly if I used a 1.4x converter to give me 450mm instead and is the problem that I am losing 2 stop of lights when I use my 2x.
Or am I missing something about this lens???