I stole borrowed some ideas from Bill Fortney's blog (www.billfortney.com) and purchased an old (1980s) Nikon 300 f/4.5 lens.
Works great with focus peaking and a $25 Fotodiox adapter. Gives me a 450mm FOV and more reach than my 55-200 can achieve. Not perfect as my MF skills are lacking - but with decent DOF it's possible to get some good shots.
I'm looking forward to Fuji's 100-400 offering next year but until then this works. I'd love to try the Nikkor 400/3.5 but don't want to spend $1000 right now.
Couple of photos with zero artistic merit as I sat on my back porch playing with the lens and X-T1.