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I've been meaning to share my experience with that lens for a while now.
For context: last year I bought an OM-1 specifically for birds/wildlife with a Panaleica 100-400mm lens (200-800 FF equivalent focal range). It's a great camera and perfect for that: great AF, great IBIS, quite compact...
Jeremy Neipp has been using Lumix cameras for wildlife photography for a number of years. I believe he normally uses an S1R, here he gives his opinion of the S5ii.
As an S5 user myself, with a focus on wildlife it is a comfort to know that others are seeing the advantages and future potential...