Well, mine, running the latest, rooted stock 4.2.2 (I9105PXXUBNE1) also drives me nuts with its crazy RAM usage. Idling, it offers me puny 200 MB free memory at best, 300, if it doesn't hate me at the moment. I admit, I have lots of apps, but all of them are greenified.
I also use Seeder and SD-Booster to hopefully, squeeze some power out.
How could I possibly enhance its performance? And no, I'm not going to flash any custom ROMs, I like the stock FW.
I'm not using any hard mods besides apps and Xposed with Wanam, so besides these my phone is vanilla. And I factory reset my phone a couple days ago.