A lot better, eh? I still don't have much shooting time behind this lens, but comparisons on a stationary object showed the 750 to beat my D7000, which I sold to buy a D7200. The 7200 now shows better than the 750 (in my limited tests on stationary objects). Are you referring to focus, IQ or what?
I have only done a few tests , but so far here is my limited experience.
On stationary subject with good light (not too high an ISO) the D7100 comes very close to the D750 as far as IQ goes. As ISO goes up, IQ degrades on the 7100. ( I can see the D7200 equaling the D750 or even surpassing it )
Focus wise the D7100 and the D750 pretty much locked focus evenly in this scenario.
Now, the reason this lens will live on the D750 and not on the D7100. Focus speed . Locking focus tracking birds. The difference HUGE! If not huge, than big.
I spent hours yesterday shooting gull. These guys are not the fastest birds on the planet, but still, they are quite quick changing directions and diving .
I first gave it a good workout on the D7100.
The weather was not very sunny but it was not badly overcast. On a 1-10 scale, I would say maybe a 6 or 7 leaning towards bright.
Now before I continue, I must say that I have some experience tracking moving objects and have become pretty good at it using the 70-300mm lens. This of course is a much heavier lens , but I still did fairly well keeping the focus point on the bird and close to the eyes.
On the D7100 it was somewhat of a problem. I was tracking the birds with the focus on the bird but it kept wanting to focus on the sky instead. I'm not saying that it was really bad, but I did have to refocus many times to get the bird and sometimes by the time it did focus on the subject the bird was already past, showing it's backside.
On the D750 it was the opposite. As soon as I hit the bird with the focus point it snapped right to it. Everytime!! As long as that focus point is on your subject it will focus almost instantaneously.
My overall conclusion with the limited experience I have ,is this.
If you're only shooting stationary subjects, this is a fantastic lens whether be on a D7100 or a D750.
Tracking birds, the winner is the D750 hands down. I will now be shooting landscapes with the D7100 and BIF with the D750. (This is not a lens that you want to be changing back and forth all day.)
I'm willing to sacrifice reach over quality and speed.