SM-T825 LTE should work with the same firmware as SM-T820 WIFI version, for example if mobile 3g/4g connection is not required. Some Samsung firmware surgeon could shed some light into this.
well like Alpaca said, the firmware of the 820 and 825 are very different. Even though there are basically no differences (other than RIL), Samsung might have totally changed the file structure and added 10k more symlinks to support the modem and RIL and integrate it with the kernel properly. Only God knows what it actually is