Hi Mcmaze, just cam across this post and will offer some thoughts. Just don't understand why the Sigma delivers too soft. I use a very old Sigma 170-500 and on most occasions it is fine, and when it is not, usually it will be me and my crazy settings or set-up. You really will need a longer than 300mm lens especially if you want to capture birds. One visit to the Kruger a few years ago, I left the 500mm Sigma accidently at home, and to this day I regret that mistake, as some of the best sightings were had, many of them just to far away for the Sigma 70-300 I had to use then. Definitely try, if can, to come with the 150-500. If you have the time, I would have both the camera and lens checked out. My experience of teleconverters is that you tend to lose some IQ, and that is a factor.
The Lens had pretty acceptable reviews and some of the images are quite fantastic -https://photographylife.com/bird-photography-with-sigma-150-500mm-os. There is also a review on Facebook if you want to explore that. And this article confirms you impressions, but not something that cannot be fixed with a bit of PP -
Bird Photography with Sigma 150-500mm OS - Photography Life
Having said that, it is also wise to remember when it comes to places like Kruger, its a case of "right place-right time" and you can get pretty close. Safari's will be an added bonus as they operate with plenty of help from other operators, and can usually locate sightings with ease.
Hope you have a fab time.
Noel