what about a 70-200 1.5x f/2.8 ??

bigfatron wrote:

I had the now-discontinued 100-300 f4 EX Sigma lens and that was physically big, weighed just shy of 1.5kg and was plenty enough to lug around all day. I couldn't see the lens being proposed being any lighter.

And to add my two cents I don't see the point of having the bulk and weight and complexity of an inbuilt TC + mechanism on a lens that (relatively) short. If you're going to build a 300mm f4 zoom then skip the TC nonsense and build a 300mm f4 zoom.