Hi Papa J,
I think you need to evaluate your standards of what's accepted noise in your photos. Or maybe you could post some examples in order to better understand. I would love to see what your noise level looks like.
What I can say is the D750 does a very good job at high ISO. I am very pleased with my D750, and I upgraded especially to get better results when using high ISO. I must say that I have become aware of the fact that using high ISO and then zoom inn 100% on a face, that will show lot's of noise. But again , I was not shooting to get the face in perfect quality, I was shooting the scene, and that looks totally different and you barely see the noise. Should I look for the face photo, then I would got closer and surely would managed to get way better quality with the same ISO setting.
I shoot a lot of photos indoor, ice hockey, soccer for kids, some DJ parties, and I shoot with high ISO. The results are very good for my standards.
Noise it will be visible the higher you go, but far better than what I could get out of my D7100. Depending on the type of photo you take some noise means nothing. Check out my posts where I've attached pictures with all exif data. You will see I use quite high ISO on many pictures. I never use Auto ISO. After long time now learning manual mode, it's seems I rarely use other modes. Also I've turned off features like noise reduction for example. I'll do that myself.
My most recent pictures are from kids soccer indoor from two weeks ago, (have not posted them here yet bu I will). This time I've managed to get pretty darn good pictures, and I used ISO 1250-4000.
One thing I have to mentioned is that I do post process all my photos since I shoot raw, and noise reduction is often applied to my photos.
Here are some links to my posts from here where you can study some of my photos as referance.
ISO 1250 cloudy day (some dogs) :
http://www.uglyhedgehog.com/t-437428-1.htmlISO 4000 indoor (lego figures)
http://www.uglyhedgehog.com/t-434275-1.htmlISO 1000 indoor (ice hockey)
http://www.uglyhedgehog.com/t-428662-1.htmlISO 1600 outdoor (my kids and dog)
http://www.uglyhedgehog.com/t-420308-1.htmlOf course , if this level of noise is far from what you would accept, than I fear you may have too look to upgrade to the bigger boys, D810 , D4.
Cheers
C.