You have the VR Adam ....... mine is the last of the "old" non-VR ............ 90% of the time I'm on a tripod ....... 10% bean bag ............. I have a D750 and D810 ........ I don't really worry to much about highish ISO's, but try to keep it below ISO 2000 especially for shots that I know I will crop heavily .......... just looked at my data .......20% were at 1/800th and 50% at 1/1000th/1/1250th ......I don't think that I knowing go below 1/800th and always try to be above ..... ISO is all over the place, but in the range 400 to 1600 ..... and up to 3200 ...... but as I said .. tripod mounted.
When I am taking "birds" with the 600, 50% of the time I have the 1,4TC attached ....... and most of the time I have the 300mm f4 PF with me on the other body to use hand held ....... I find the 1,7TC works well will the 300 PF.
I'm sure that you will love the 600 ... but as I have said before ...... IMHO it's an additional lens for "birding" as it is quite restrictive because of it's size.