My current set up is all-Canon with 7D (original) with 100-400L (original). I am looking at different upgrade paths and stalled myself to a standstill:
- 7D (original) and buy the 500mm - this is the gold standard and not sure I can stretch that far
- 7D (original) with 100-400 Mk II
- 7D (original) with 300mm f2.8 (original) and 2xtc (MkII) - I could do this if I really, really wanted it
- 7D with Sigma 120-300f2.8 and 2xtc - another one I could push myself to
- 7D2 with my existing 100-400 (original)
I have always believed in 'glass first' but the improved performance of the 7D2 sounds so tempting and would be equivalent (shutter-speed wise) to taking the 100-400 to f4 at 400mm.
Decisions, decisions....
It is difficult to say which would be the best option, I know which is best for me, but that doesn't mean that it is best for you!
1. 7D + 500 F4. This would be my choice. However you will have to budget for a decent tripod and head and remember that it is heavy. Also you will have the same ISO limitations that you already have. Against that the Canon 500 F4 L IS is a simply superb lens that will be more than up to future upgrades.
2. I haven't used the 100-400 Mk2 yet but I am reading awfully nice things about it! Whilst I have no use for the IS or zoom functions the improved IQ and close focusing look very tempting.
3. 300 F2.8 L IS + 2 x extender Mk2 - er no. 300 F2.8 L IS + Canon 2 x extender Mk3 = yes. The Mk3 extender is far superior to the Mk2, my Mk2 was just a waste of money. The Canon 300 F2.8 (any version) is a superb lens (probably why I have one!), mine is the L IS Mk1. I have tried the Mk2 and it is better but the Mk1 is so good that I didn't feel that the Mk2 was worth the extra. My only concern is that you will end up using extenders most, if not all, the time and whilst it works very well with extenders this situation is not ideal.
4. 120-300 Sigma F2.8 + extenders. This has the versatility of being a zoom but your IQ and AF will not be as good as option 3. The Sigma is a great lens but the Canon is just better but without the zoom function.
5. 7D2 + your 100-400. Cheapest option and the original 100-400 can be very good - mine was. You will have more advanced AF and better ISO performance. It is debatable how much the ISO improvement is but it seems to be about 1 stop - so pretty useful.
For what it's worth I mainly shoot small birds or larger ones further away so, to me, reach is everything. This leads to very heavy backpacks and very light wallets! If you need reach and mobility then the Canon 100-400 Mk2 looks very tempting as it appears to have very good IQ and weighs next to nothing. Alternatively the 300 F2.8 L IS Mk1 is heavier but simply stellar at 300mm, extremely good at 420mm and very good (in good light) at 600mm. If mobility is not an issue then sell a kidney (that's why you were born with 2!) and start looking for a good used 500/600/800. The 600 F4 L IS appears to be the best value at the moment but it is a bit of a beast and a challenge to shoot hand held!