For the 26,500 shot at 1/1600 , base iso would give a shutter speed of 1/6th shutter speed
I've noticed that FZ1000 in-camera default NR at higher ISO's (800+) takes on toll on smearing image's finer details (e.g., hair) as posted by others and myself in "
In my experience" JPG NR posts.Below is one of the images from my
post on my experience with FZ1000 JPG NR at higher ISO's:
Just PP did some interior church pics from last Sunday, JPG+RAW (Standard Photo Style/Default settings; AWB) shots that ranged from 800~1600 ISO and saw the "mottled" you mentioned in fine details especially in people's hair shown below at higher ISO's (blurred portion of face for privacy reasons):
View attachment 979655
For a little more exploration I downloaded some JPG+RAW images from ImagingResource.com FZ1000 Review at different ISO's. Image below are a side-by-side comparisons of 125 ISO JPG, 800 ISO JPG, 1600 ISO JPG, and 1600 ISO RAW I processed with DxO Optics Pro 10 shows the finer details in the fabrics lost as the ISO increases. As one can see there's quite a difference in the fabric details between the FZ1000 1600 ISO JPG and the PP 1600 RAW image.

Bottom Left Image: OCC JPG 1600 ISO; Right: PP RAW 1600 ISO
For those that may be interested images below are the JPG OOC and the PP RAW images both reduced 50%; just viewing the initial first display size after clicking the "view original size" link can easily see the differences between the two images.
No surprise there's more detail in the RAW images, Jon, but might be slightly more detail in the jpegs, if NR was lowered.
Plus I find the
Natural photostlye to have less processing at defaults than
Standard photostyle. Add in minus for sharpness, contrast and NR and the camera has less smearing of fine details and one can adjust in post a bit better, than if using default settings..
I am still experimenting on just the right settings, and choosing to use FastStone Image Viewer as my post production software.
Otherwise, I will have to spend some $$ on either PhotoShop, Lightroom or DXO Optics Pro, or other software, and then take some time to learn it.