I'm curious, what do you like in the ROM that doesn't exist in Xposed?
It is true that many of the tweaks available in CM exist through Xposed. However, is rather have everything in one place, than installing 10 different modules, which can conflict with each other. Additionally, as I mentioned, Xposed for Lollipop is still in alpha, and not all modules work.
Also, to the best of my knowledge CM is very from AOSP, as a matter of fact Stock Android is much closer to AOSP than CM. AOSP stands for Android Open Source Project, that's core Android, all ROMs (Google's "stock" included) base their version of Android on AOSP.
CM is based on AOSP, and is very similar. The only stock ROMs that are closer are Nexus ROMs, and perhaps the newer Motorola ROMs. Every other manufacturer heavily modifies every aspect of the OS, and their stock ROMs don't look anything like AOSP.