Thanks. Since I've gone all FF I really don't need my 300 f/2.8L I IS anymore. What I was thinking is selling my 300 v1 and 400 v1 and just buying the 400 v2. I will welcome the loss in weight since many of the facilities where I shoot require a fairly long walk from my car and selling both lenses will match the funds required for the v2. Still thinking about it though.
Personally, I'd sell the 300 and 400 v1 to get the 400 v2 - especially as you are not likely to be out of pocket.
I'd also suggest looking at meta data (LR) for last year or so for all the shots done by yourself to ensure a large % were not taken at 300mm. If not get the 400 V2.