You are among the few that manage it, congrats! I know a few with top end i7s and brand new 4090s that can only get a steady 60 FPS on Ultra, with some dips below.
I have done some tweaking for my 3080
DLSS wouldn’t improve performance that much for you in regular play, especially if you are using Vsync for your display (you should be) which will restrict the frame rate to your monitors refresh rate.
But you would likely see an improvement in rendering and some performance benefits in resource intensive instances like big set piece battles.
I have a 100hz ultrawide monitor myself but, unfortunately, I have never gotten close to 100 FPS without going all the way down to low, at least on my ultrawide. My system isn’t cutting edge, but it is well above the recommended specs for Starfield (i5 12500x, RTX 3080ti, 16 GB DDR5, High TP Gaming SSD, etc.). Some of it is the render resolution, as I can get near 100 FPS just playing on the 16” UHD laptop screen. But, for comparison, I can run Jedi: Survivor on Ultra and sustain 100 FPS regularly on the ultrawide.
It’s just a Bethesda trademark poorly optimised game. It’ll improve with updates and native DLSS support.