Default [Q]KitKat Optimizations

So it's been a long time since I've been in here, and I've got a (rather stupid) question that I'm not sure can be fully answered, so I apologize in advance.

I'm looking to completely wipe my (now old) I777 and repurpose it as a music player, and I want it to be as fast as possible (so I don't feel totally bogged down when using it next to my shiny new S4 xD). Best ROM questions are bad, so instead I'd like to ask: does 4.4 KitKat actually bring any useful memory/resource optimizations to our phone? I understand the main thing the update brings is the ability to scale back the OS's resource usage so it runs on lower hardware and I was just curious if that actually happens with our phone on any of the current 4.4. ROMs, or if they take about the same amount as older JB ROMs. I do know all of the current ROMs, whatever version, seem to all run pretty fast already, but I just want to make sure this experiences as little hiccups as possible while still running the latest software and apps.

One last question: since I started flashing, I've used Sammy-based ROMs exclusively. Can someone remind me what functionalities I lose switching to an AOSP-based ROM? I remember Samsung's camera app (which usually has more options) is unsupported, as well as MHL-HDMI output is broken, but is there anything else?
Current Configuration
Phone: AT&T GS4 (SGH-I337)
ROM: Stock [4.2.2]MF3, rooted, manually debloated, and lightly themed using Wanam Xposed v.2.7.4
Kernel: Stock 3.4.0 kernel
Modem: I337UCUAMF3

Dedicated Music Player: AT&T GS2 (SGH-I777) running [4.4.2]SuperNexus 4.0 (Build 1) with included RenderBroken CM-11 Kernel (2013-12-17 Build)

Avatar Image taken from Karen Hallion