You are correct. Use offset mode if you want to continue to allow the CPU to vary voltage according to demands. Use manual voltage and it will remain constant.
Just a guess, but with a 1.29V you can probably jump to 4.2,4.3 without any other changes, possibly up to 4.5 with some minor tweaks. I'm running 4.2 on all cores (up to 4.4 on fewer) with a -0.20 offset at the moment, this idles down to around 0.85V and peaks at a bit under 1.2V when loaded. VTT at 1.016 and VCCSA at 0.825 with 64GB @ 2133.
Rampage IV Black Edition | 4960X 4.5 GHz x6 @ ~1.232V | 64 GB 2133 1.5V @ Stock XMP
2x Crossfire Matrix Platinum 7970s @ Stock | AX1200i | H100i | Corsair Link Commander
1 TB EVO | 3 TB WD Red | CoolerMaster Cosmos II
