Hi Malcolm,
I think this is a great idea, and have often experienced cases where the thread shown is unrealistic.
Note there is a workaround way to make the thread display bigger, just not smaller.
Cosmetic thread textures use a SmartPaint decal, so you can use the mapping tool to adjust the projection of the decal.
It's important to note you need to select down to the face level to see the Decal mapping tool available.
I've attached a Gif.
Note, I think your request is still needed, this is just a workaround that half-works (only increases thread size not decreases).
You could also go track down that thread texture if it was really needed and apply it as texture not decal so you could do thinner threads.
Then save that graphic to the catalog and drag it out onto anywhere it's needed.