I recently fitted some DBA 5000 2 piece discs to the Senator and was pleasantly surprised at the weight difference.
My Holmart AP replicas weighed in at 11.5kg while the DBA’s only 9.2kg, an unsprung weight saving of 2.3kg.
So while I was in the mood I weighed my 8″& 9″x 18″ BBS RE’s with 235 & 275 tyres against my 8″x18″ spare (VY R8 wheel with 215/45×18 Pirelli).
The Combined weight of the spare and tyre came in at 25kg, while the 8″ BBS/235 tyre was 21kg and the 9″ BBS/275 tyre was 21.5kg.
A healthy 3-4kg in unsprung weight.
I know in the real world it doesn’t mean much to driving feel but less unsprung weight gives better/longer life to suspension parts and hub bearings.
Every bit helps but as you say you probably wouldn’t notice it on the street. I would say the benefits in regards to longevity would be marginal as well given all the people running with 20 inch rims and such that seem to have no issues. On the track where you would be pushing hard would be a different story.