Sigma 300mm F4 APO - sensible teleconverter?

Hi all

I've got a Sigma 300mm f/4 APO that I use on both a K30 and K5, but I've never got my Pentax 2XS teleconverter to play ball with it - the aperture coupling never works as it should.

I've seen a Sigma EX 2X teleconverter for sale for a decent price which I'm tempted to try, however I thought 2X was maybe a bit strong and I'd be better holding out for a 1.4X. I'm not sure a 600mm f/8 (if I have that right) would be that easy to handle or compose, that quick to focus or all that sharp, although the idea of using a Sigma branded TC with a Sigma lens would help with the connectivity.

Worth a look, or go for something else? It'll be used for wildlife, which makes me think that such a slow overall aperture wouldn't be ideal.

As an aside, I've not had the most consistent success with sharpness from the lens, some images it's pin-sharp and others the focus isn't too clever, so I'm wondering if sticking a teleconverter into the mix is going to cause more problems.

Any advice gratefully received!

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