Well when I had my GS4 GPE it too was dramatic the difference between it and Touchwiz edition I had prior. So yeah the rom does indeed make a difference in the same hardware.
I like Touchwiz launcher myself (or at least many of the extras over stock), and just hope they do what is rumoured and streamline it - everyone benefits then. But the cynic in me thinks if you are streamlining something so dramatically why do you need to throw 4gb ram at it. So one of the rumours is wrong, and in this instance I hope they are wrong about the memory.
Out of interest, How much memory is free with Cyanogenmod installed on your Note with your usual apps all open in the background ? My Moto x always seems to have about 1gb free and that's with system using around 250-300 and my 10 open apps occupying 500-700mb. This is with a 2gb device only. It was a similar story on the HTC M8 with Sense 6. So I would assume the Note 4 would sit with well over 1.5gb free even with many apps open.
Hence again more of a reason why I don't see the point in 4gb rumours for the S6 currently.
It probably varies wildly, but right now with 5 or 6 apps open I have 1.2gb free, with the system using 348mb.
I still cannot believe how fast CM12 is. Instant everything, instant in every sense of the word. The most eerie thing is hitting the recent apps capacitive button and truly having all open apps pop up instantly no matter how many are open.
I previously commented on how much faster the T-Mobile variant was than the ATT one, even completely stock and unoptimized, leading me to believe ATT bloatware played a large part in its slowness. But this is another level on top of that. It's a shame there is no GPE version of the Note 4 as I now know I wouldn't hesitate for a second to choose that one.