I've filled out the feature request about multi-processing.
And I also do a lot of C4D, so I can understand your situation where C4D can fully utilize your CPU but AE is just using one or two cores. It's very upset.
As for the memory setting, I reserve only 3GB RAM for other apps because when AE is rendering, typically I don't do heavy work on my computer. I just get my Chrome Browser on, and play some music, doing this kind of stuff. In this case, reserving 8GB for other apps seems kind of waste.
I want AE to use my RAM as much as possible, as long as it's good to him.
So I leave 3GB for other apps
Cheers,
Aaron
The reason the recommendation is to leave more RAM is that some of the stuff AE does is considered system processes. The camera tracker, warp stabilizer, and several other tools (as well as some third-party plugins) use system resources other than what AE reserves for itself. I have seen issues where too little RAM was left and someone tried to stabilize a shot and it would crash or take forever.
You've got enough RAM that having all that RAM dedicated to AE won't speed up a render anyway. In fact, it is has the potential to make it worse instead of better.