Are you sure the mod you're using is injecting DLSS3? Using the DLSS 3.5 DLL is not related to that. If it's only injecting DLSS2, it would be entirely expected to see negligible performance difference vs. FSR2.
"DLSS2" and "DLSS3" are not references to the version of the DLL. They are references to different output modes of the DLL, which is also backwards compatible, i.e. the 3.5 DLL can output either mode. The Starfiels mods that were available immediately at launch do not use DLSS3, or by extension frame generation. It doesn't matter which DLL version you use
Well that's what I'm saying. My 3090 can use 3.5 but it can't do the frame generation feature that 3.5 is capable of, therefore the performance gain is negligible over FSR2. The mod I used did support DLSS 3 + frame gen.