I am confused by this discussion, so before I try to sell the item in the picture, I'd be grateful if somebody would put me right if I'm wrong.
I think it's what you call a quill to ahead converter, not an extender. I don't understand the questions about tensioning the headset because I assume this works with a threaded headset and fork, which are already tensioned. This then fits in like a quill stem and you clamp an ahead stem to it. You don't need spacers, but you can use some if it makes it look better.
If you have a threaded headset and fork that you're happy with, one of these would give you the function of an extender without the cost of a new headset, wouldn't it?