According one of the reviews, the K-3 is faster than the D7100 for focusing, but the D7100 is slightly better for tracking, personally, those are the only two systems I'd consider.
I seriously doubt you'll find a better system for what you want to do.
The images on this page show you what the Pentax can do…
https://www.pentaxforums.com/forums/122-lens-clubs/55946-300mm-plus-lens-club...enses-683.html
Here's sampling of my own work with an A-400.
2013-12-13
Camera: Pentax K-3
Lens: Pentax SMC A-400mm
ISO- 800
Aperture: ƒ8
Shutter: AV mode, shutter speeds 1/1000 1/3000
The day starts shooting through the kitchen window , birds out in the maple trees apparently eating seeds.
And a woodpecker on a tree.
Then moving outside. It's minus 20 today, even in full outdoor gear, it was chilly.
The male Purple Finches went straight to the feeders, I couldn't get any pictures of them anywhere else.
The females were more co-operative…
We also had a group of Goldfinches there as well.
And even a rodent.