From what i can tell with my BIF subjects no mirrorless camera would be a lot of good,i dont get any warning though where i go they just appear over some trees, then in seconds are gone over some others,as you know my Tamron is more than likley going back.After nearly a year with it i dont find 150-600 the best for BIF pictures,most but i will admit not all the time its back to 450mm or less to stand any chance of picking the flying bird up and relocating it if i lose focus lock.
I am fairly sure i will forgo the extra length and get a 300 plus 1.4 this is why i bought the P610 i can take it on the other shoulder and have immediate access to wide angle through to telephoto when 300mm or 420mm is totally wrong.its like having a zoom lens without having to change lenses.
Nikon bill it as a cover all camera but its far from that, in my mind it is just another lens with its own limited use.