Ultrabookreview.com's measurements estimate 4h for UHD and 6h for FHD, which is a bit more realistic for moderate productivity work at medium screen brightness. Dave2D is also known for his realistic battery life testing.
Unlike Lenovo that have sourced large batteries since before ULV Haswell, Dell has been working on power saving features for a longer time. Primarily with XPS and Latitudes, they constantly experiment with CABC, LP memory and displays, auto on/off touchpad, capped SSD speeds, I/O power capping and such that have benefits but causes some reliability issues.
Firmware is not likely to noticeably improve battery. Instead, it is recommended to tweak native power settings, use 3rd party programs like Throttlestop or Intel XTU, and have at least one battery monitoring software installed. Also, manually observe power draw throughout the workflow and check which usage or background apps behave abnormally and draw more power.