Looking to increase my reach occasionally. Will adding the 1.4x III to my 300 f4 yield as good (or better) results as the 400 f5.6?
I would think much would depend on how important IS is to you. Under perfect conditions I would very much imagine the 400mm would have better IQ, and I would think better AF action.
You don't mention what bodies you'll be running this glass with. But you do say that you'd like to increase your reach occasionally
. The +1.4III is going to be a lot lighter on your wallet and a lot lighter in your bag. I had a 300 f/4is which I occasionally used with the +1.4II and from memory the results were perfectly fine. Mind you, I had a stellar copy of the 300 f/4is which I'm sure helped.
Why don't you get the +1.4III and see how you go. If you find that 400 (420mm actually) is a focal length that fits your shooting style and f/5.6 just isn't cutting it, that may be the time to take a deep breath and go for a real 400, the 400 f/2.8is. I would expect reasonable numbers of 400 f/2.8is glass to be finding it's way onto the second hand market as sports and some wildlife/bird shooters trade up to the new 2kg lighter 400 f/2.8II.
A pre-owned 400 f/2.8is is a risk free purchase. If you find you're not using it, re-sell and you'll be unlikely to lose a penny on the deal.