Hey folks,
Been reading the internet back and forth for a while now on this question, but figure that it's best to ask people who actually use these lenses, rather than trust reviews on YouTube etc!
I'm currently using a Canon 5D MKIV for most photography, and my trusty old Canon 70D when I need that extra crop-sensor reach. I mostly photograph Wildlife and Airshows (I know, weird mix), with a bit of everything else in-between! 
I use 2 lenses mainly:
1. Canon EF 70-300mm f/4-5.6 L IS USM
2. Sigma 150-600mm f/5-6.3 DG OS HSM Contemporary
The Canon is great for quality, and the focusing is fast enough for most of what I do. The Sigma is also capable of some great quality shots, but it's heavy and I find the autofocus a bit slow for fast passing jets etc. I find that once I lock focus, anything fast moving has probably already moved away from where I wanted to take the shot. As I said, I have some GREAT shots with the Sigma, but mostly on stationary targets, and some "more luck than judgement" fast moving planes and animals.
I was looking into adding a 100-400mm lens into my kit, for that extra reach over the 70-300mm Canon, and also because I already own a 1.4x iii Canon teleconverter which isn't compatible with the 70-300mm (well, it does work, but only over 250mm, and it's risky to use). What I really want is something that focuses fast, produces sharp images, and isn't the crazy weight of the Sigma. And reading around, the 3 main options seem to be:
1. Canon EF 100-400mm f/4.5-5.6 L IS II USM - This was always the one I wanted, given how much I love my 70-300 version, and reading around I know it does sharp images and focuses fast. Price is the only issue, but as it's been around for some time now, you can pick up a 2nd hand one for around 1500 USD/EUR now. I also have the 1.4x iii teleconverter already
2. Sigma 100-400mm f/5-6.3 DG OS HSM Contemporary - Sigma has always been my 3rd party brand of choice, but I worry that the autofocus might be slow like the 150-600mm, and maybe I'll get less "sharp" keepers then I would with the Canon. Also not sure about teleconverter compatibility? It works on the 150-600, but the images aren't that sharp at full zoom I find
3. Tamron 100-400mm f/4.5-6.3 Di VC USD - I admittedly haven't used Tamron before, so I am a bit skeptical. But it reviews well in this category and, like the Sigma, you can get it new for a lot less than a used Canon, so it's on my list.
Does anyone have experience with all 3, especially for Airshow shots (or I guess birds in flight as it's a similar style). I am trying to find "real-world" feedback as opposed to YouTube reviews that might have an element of bias. Assuming that budget was no problem (I mean, it is, but let's assume not for these 3 options), what would people go for?
Thanks very much for any advice. I did search around and didn't find anything specifically about these 3 lenses, so I apologise if it's been asked before.
Cheers,
Orias/James


. As you suggested, I was looking at various 2nd hand options, and I managed to pick one up for €1300 in the end (~ $1400 USD). I did see the ones for sale on the forum here but, unfortunately, they all seemed to be for sale in the US. I am based in Europe so, even though I paid about $100 more than the one on the forum linked above, by the time it got to me I would have no-doubt been stung for significantly more than that in import tax and shipping costs. 
