Just thought I'd update this.
Managed to drop the CPU to 4.5Ghz at 1.265v and it still seems stable and temps have dropped a little - 72 max core, 77 max CPU package.
Also managed to set the cache at 3.9Ghz at 1.159v.
Is there a difference between Adaptive mode vs Offset mode? I have the CPU in Adaptive and Cache in Offset but they both still down clock/down volt at idle.
NZXT Phantom 530 | EVGA 850 G2 Supernova | 5820k @ 4.5Ghz / 1.258v | Cache @ 3.8Ghz / 1.084v | x61 Kraken (Silent) | ASUS X99-A | HyperX Fury 16GB 2400Mhz CL15 @ CL12 | LG 34UC79G / Curved / Ultra Wide / 21:9 / 2560 x 1080 / 144Hz / 50-144Hz FreeSync | Gainward Phoenix GTX 1070 | 436.51 Drivers | 1506 Base / 1683 Boost | +210 / +615 | Samsung 850 Evo 250GB SSD / Seagate Barracuda 7200RPM 3TB HDD | Custom Windows 10 X64