Russ I sort of wonder whether your reservoirs are the best place for a budding bird tog to cut his teeth,it all depend on how tame the birds are?? I asked about the 400 because I'm lucky to shoot with one myself,mine being f4 opens the door to the 2X as well. That isn't an option for you bar maybe tricking the camera with a kenko??????
For me, there are going to be two big limits to an F8 combo ( I haven't used the 7dii but it would apply to any I guess) ,being stuck on center point,is something I personally hated. 2nd the amount of light needed at f8. We are not always blessed with lots of light here often shooting in lower light especially birdz and beasties.
Russ reach is cool,but you want to be close to your subjects,if humanly possible..bar the bird scape types of images,naturally. Find some tame stuff have a bash at gulls for your muscle memory with birds in flight. I'd honestly star with the bare lens especially with BIF tracking gets progressively harder the more reach one has
Mate i'm not trying to put you off : if you are going to tackle those watery places get a little chair you can carry try and pick a cool spot pref sun behind you set up and sit still, the lower to the ground the better. It sounds like you have little chance of stalking to get closer with no hides low and wait may be a good tactic,give it an hour for the brids to start to forget about you. Mate this time is never wasted,you will learn pretty quickly which birds like which places on your patch you'll start to see favoured spots where they come and go from the lake...flight lines.....the more your learn your little patch the more you learn your subjects, the closer you'll get and the more the chances will come!!
Your water fowl flight lines will strengthen at dawn and dusk they move at these times of day from food to roost, may well worth trying to exploit.
So yes the exts are worth the bang for the buck but they come with some limitations.Reach is cool but again limitations Knowing who does what and when helps all ways
all the luck maybe slightly left of the reply you are looking for it might help
stu