Hey, I messaged my buddy and he sent me this link. It shows his experiments with some radius reduction.
https://www.reddit.com/r/elderscrollsonline/comments/4a4b3q/tg_easy_mode_how_to_master_sneak
If you don't want to use the link he concludes
"You can get a total of 9.45 reduction in stealth range, effects shown in pic.
It's far more pronounced if you run past people, sometimes i can bump into them without them noticing.
3 reduction from being a sneaky race (passive)
2 reduction from the nightshade 3 pc set bonus (vet 6-9 ish)
2 reduction from the night mother 5 pc set bonus (was 25-29 ish, now leveled to you and inc jewellery).
35% reduction on that from medium armour.
And with all the associated sneak cost reduction you still gain stamina while moving in sneak."