1600mm - Too crazy, or usable?

My wildlife photography setup currently consists of a Canon 200D + Tamron 150-600. 
Here's what i'm thinking for my next upgrade: A Canon 7D mk ii, Sigma 500mm f4.5 EX DG HSM, and a 2x extender. With the 1.6 crop factor of the 7d, that gives me a 1600mm focal length (No need to complement my math skills). Will the images somehow be affected at such long focal lengths? I know in hotter climates the horizon can sometimes become a bit "blurry" or "wavy", almost like what you see behind an airliners engine. I live in the UK - so that is rarely an issue, but perhaps it would get amplified at such a powerful focal length? I'm fully aware i'd be stuck with a maximum aperture of f7.1, but i'd only use the 2x extender in optimal hide conditions. Any comments, or recommendations are appreciated. 
This is my first post on a photography forum, apologies if I've done something wrong!
~Campbell
 

As for the math, you don't have to attribute any advantage
at all to the use of a cropped sensor… there is none!

The smaller sensor doesn't get you any more focal length. Nothing changes the physical attributes of a lens.

What you get is a different (narrower) view angle. The pixel density of the sensor will provide more detail because there are more of them in a given space. But that's about it. That whole 1.6x thing? Forget about it.

The 2x extender will impact your image quality, possibly quite a bit. The Nikon 1.4 will take an image to 80-85% of original IQ and it's noticeable. Do some googling, but I think you'll find the 2x may not give you the results you need.

I'll also point out here that the advantage of your (alleged) 1600mm over 1000mm is a 60% bump. (1600/1000 or 1.6). In other words, it's not as significant as cramming more pixels onto that sensor (which is where your real improvement comes from).

You might want to really think about whether that 2x TC is going to pay for itself.

garyrhook wrote: The 2x extender will impact your image quality, possibly quite a bit. The Nikon 1.4 will take an image to 80-85% of original IQ and it's noticeable. Do some googling, but I think you'll find the 2x may not give you the results you need.

The Nikon 1.4 only degrades the IQ that much if you're using a meh lenses to begin with.   With a 300mm F/2.8, the reduction in IQ is hardly noticable. 

That being said, even Nikon's TC 2 III degrades the IQ even on the 300, just not a ton.  Although that can be helped by stopping it down a little. 

That being said, all TCs effect AF which is important to remember. 

That being said, I'm not sure if getting a 2x TC would work for the OP.  TCs are nice but they're far from free "focal length."  They're a trade off of IQ(how much depends on the lens), AF speed, deduced light/aperture for added effective focal length.  I like my TCs but they're not a magical tool that just adds focal length. 

the four vignettes wrote: That being said,

Is there an echo in here?

garyrhook wrote: Is there an echo in here?

Well, you were fairly off with your IQ degradation percentage for the 1.4 TC...

Teleconverters are damaging to image quality.  You shouldn't use them in my view.  With the modern high res digital cameras we have today, you will do better cropping the image than using a TC.  The fixed focal length telephoto will certainly outperform your zoom lens so that is a true upgrade,  I used a 500mm f4 back in the film days and it was very effective for wildlife.  Cropping the image to the field of view of the 600mm focal length will cause no noticeable image degradation. 

I approve of the upgrade to the fixed 500.  I recommend against the TC.  Others have already explained that using the lens on a smaller format sensor doesn't provide any more telephoto power.  It simply crops the image.

Not all lenses are created equally, and not all of them respond the same to TCs. My Sony 200mm 2.8 looked horrendous with the x2 TC AND the 1.4, but is it a "meh" lens? Hell no, not even close. My 400mm 4.5/5.6 looks better with the 1.4! So, I ended up selling the 200, which makes me sad er'day, but it just wasn't as versatile as I needed since not being able to use the TC. 

If IQ is not the first thing on your list, TC away, but, the trade offs can be difficult when dealing with less than awesome lighting. I would find others with the exact set ups you're interested in and seeing their results. IMO, TCs are too unpredictable without renting or seeing the set up from someone else. That's a lot of cash to put up and have it fail, like my situation, and that stung quite a bit.

On a final note, have you ever seen these amazing wildlife shots that are so perfect, you wonder how in the world they get them? Most of them are using prime lenses and/or setting up in gilliie suits right on top of a perch next to a water or food source. Then they wait for the perfect shot. Trying to get close to a lot of wildlife with just the lens itself and getting those amazing shots is very, very tough otherwise. 

If you are going for aircrafts, I would consider a full frame camera with x1.4 on a 400 or 600mm. As long as the skies are clear, you should have good lighting in an airfield situation. Many of the full frame cameras have phase detection points better designed for action and can potentially track better, even in lower light.