Oliver, you're right in principle that more ram should enable you to use more processors, but the truth is that AE often is unable to hand multiprocessing efficiently. As you know, depending on the project, it's sometimes better to have multiprocessing disabled entirely, but even in those situations where it lends some advantage, it might still not be better than having fewer, faster processors.
If you post a link to a collected test project and your render times (both nMP and iMac), I can try rendering it on my system to see where the bottleneck is.