Are people comparing apples with apples here;
R26R is the lightweight, part stripped Megane, and it weighs 1230 Kg, complete with a weight adding and chassis stiffening roll cage.
S3 weighs 1455 Kg (both according to the manufacturer)
Renault removed over 135 Kg from the R26 to ger the 'R', as they added weight back in with the 'cage.
So you have to remove 100+ Kgs of 'stuff' from the S3 to get to basic R26 weight.
To get to R26R weight you have to remove 225 Kgs...
...not going to happen.
Personally I think weight saving is a great idea...but sadly an S3 isn't the best starting point as they are needlessly heavy as hell to start with.