Become a MacRumors Supporter for $50/year with no ads, ability to filter front page stories, and private forums.
I have a 6s Plus and an Air 2.
The performance is pretty much the same.

I did a 3dmark benchmark (complex test) and the 6s had better framerate (8-10fps), but that's to be expected. A8 is from 2014, and the A9 from 2015. Also, the iPad has a higher res display.
 
It will be

iPad Pro 2
iPhone 7
iPad Air 3
iPad mini 5

in the future?
Who knows. It will depends on the SoC inside.
Some patterns to observe:
1. AxX will always be faster than the non-X Ax version (A9X is faster than A9)
2. The Ax on an iPad will always be faster than the Ax on the iPhone. (A8 on iPad is faster than A8 on iPhone)
3. An Ax+1 can be faster than an AxX. (A9 is faster than A8X)
 
Register on MacRumors! This sidebar will go away, and you'll see fewer ads.