The opposite is true for parasitic loss I believe.
Weight wise, it's really not a big change. While every pound counts, it shouldn't count if longevity comes into play. Look up threads on the 8-3/4 vs a DANA or Ford 9.
talk to guys who raced back in the day and the two things that were common
..remove the Dana and replace with 8 3/4
..remove the Hemi and replace with 440
Yep, it is true, stupid people in every decade existed.
Ronw, the 9-1/4 should be stronger. The pinion size is nice and large. However, as mentioned, parts maybe of an issue. I'm not sure how that narrowing vs. axle taper issue would play out. I myself have never seen a 9-1/4 narrowed.
If you ever narrow it, do so document it all and keep recorded prices. Thanks