I was once like you, wanting to find the most light weight firmware, but that was habit after previously having a Galaxy 3. (128mb of ram, 40-80mb app space, low spec'd).
Instead I'd just go for a well optimized rom for the S3, say it was compiled with O3 and the kernel was well tweaked. Flash the gapps micro, disable unwanted start-up programs and remove any further system software.
I'd use a 4.4 CM based rom, download the testing branch of the Boeffla kernel for CM11, Odexed gapps micro, enable Art in developer options, do the start-up stuff and apply any kernel tweaks you like. I'm using Resurrection Remix as it has a lot of features (missing a lot I'd like), generally faster than the other rom I usually use (Archi) and has pretty good battery in comparison.
There's just so much love on this forum, it makes me want to cry.