The no-name 2x converter (it might be a Kenko) that I’ve bought second-hand is a four-element lens and it aligns with canon’s auto-focus and metering facilities. But, the auto-focus doesn’t get a grip on the desired object easily above f4, and not at all (so far) at f8.
A 2x converter halves the aperture (e.g., f8 becomes f16), so I quickly decided to go wholly manual. In bright sun, as it is today in Brisbane, I set the camera at f8, 1/160 sec, with ISO 400 and focused the lens manually (auto-focus off, IS on). This setting requires a tripod to avoid the wild shakes.
My best result was a shot of an egret at 235mm (=470mm) on my 70-300mm zoom.
It will pass in a crowd. I’ll probably use the converter for moon shots and for other slow moving, brightly lit objects. I’d not recommend it for macro work.