Cliff,
To back up what others (particularly Justin and Vespo) have said, if you crunch the numbers, your thoughts about your current rig vs the proposed Sony rig (a7rm3 +100-400f5.6 x1.4TC) are pretty much in the ballpark -
Yes you will end up with 16MP with a 35mm focal length equivalent of 910mm ..... the only difference will be that the
max aperture will be f5.6 with your current rig, BUT
only f8 with that Sony rig. This may in fact make a big difference to you, light wise hence shutter speeds. It may also provide lesser AF performance. The Sony rig you propose weighs 2.22kg (about 40%) versus the 5.33kg of your current set up.
Your rig will also have slightly (about a 1/3rd of a stop effectively, better bokeh). Even though the pixel pitch of the a7rm3 sensor is some ~20% less than your camera - though DR performance once the Sony is cropped looks pretty similar - maybe even slight advantage Sony. Put that down to a newer and more advanced sensor. You can check this out (and other possible body options such as the Sony a9, Nikon D500, or Olympus OM-D EM-1 m2, etc) at photons to photos here:
http://photonstophotos.net/Charts/PDR.htm
You could also check out the IQ using the sample tool at DPREVIEW where you can compare up to 4 cameras at various ISO settings.
I think the ultimate answer to your question (what rig to down weight to) lies with
how much weight you are prepared to carry ? , and do the finances stretch to some of the lightweight developments on the near horizon?
Already announced is a Canon 600 f4L IS III at 3.05kg.
There is also a Sony 400 f2.8 GM at 2.895kg.
https://www.birdforum.net/showthread.php?t=363772
There are strong rumours that the Canon 500 f4L IS III is due to be announced in the 2nd half of next year. I would think they could get this down to around ~2.67kg.
There is also the Canon 600 f4 DO at around ~2.5kg or so possible after that - maybe even in time for the 2020 Tokyo Olympics.
https://www.birdforum.net/showthread.php?t=317888
Nikon has already released a PF (diffractive) 500 f5.6 at 1.46kg, and it is possible we will see the 600 f5.6 version at ~1.8kg maybe later next year? We can hope sanity prevails but expect ~4000 USD or more ....
It is also likely that Sony will release an a9m2 before the Olympics, and maybe even a matching 600 f4 GM which would also be around ~3kg. Both Nikon (certainly) and Canon will probably come out with FF Mirrorless a9(m2) competitors as far as bodies go. Expect to use adapted long lenses at this stage. Needless to say that all the Canon L IS III's and supertelephoto Sony GM glass are going to be "sell one of your children" expensive ! :eek!:
One of these new lightweight "big whites" will drop your weight by 30-40% (up to 2kg) when paired with a Sony a9, giving similar focal length and resolution with arguably slightly better IQ.
Otherwise apart from the rig you proposed (with it's f8 max aperture drawback) , I think there are 2 further realistic options (staying at f5.6 max) ....
(i) MFT Olympus OM-D EM-1 II with 300 f4 PRO + 1.4xTC for 840mm eq f5.6 . This rig is just over 2 kg, very slightly more resolution, similar DR and likely IQ than your current set up.
(ii) APS-C Nikon D500(S? by then) with PF 600 f5.6 (not officially announced yet - 2019?) for 900mm eq f5.6 . This rig will weigh ~2.66kg (exactly half the weight of your current rig), have a bit more resolution, similar or slightly better DR, and comparable IQ. I think this is the path I will take - at least until delicious 600 f4 Diffractive goodliness lands and I can evaluate whether the back and wallet ! can bear the weight ......
Unless you want to outlay wads of cash for Uber light MkIII big whites, or really want to bleed for the Canon 600 f4 DO (and likely kiss one of the children goodbye :eek!: ) , then perhaps you are about to become a Nikon man ..... ??
Either the current Nikon D500 or D850 with either the current Nikon 200-500 f5.6 or the Tamron 150-600 f6.3 G2 will get you through (saving you 2.5kg to 1.75kg depending on your current rig, or ~50% to ~33% in weight) for now until the 600 PF lands .......
Over to you .....
Chosun :gh: