Performance wise, you're already seeing degradation in the efficiency of your processor by the time it hits 50% utilization.  At that point, it's like having a saturated connection and things slow way down on getting through the queue.  Upgrading would get you a lower power utilization and and smoother and more speedy execution even if there aren't any game play factors.  I highly doubt there are, getting smarter with more resources available is a pretty high end feature.

I think the game can use up to 12 cores, a Dev did say once but I am a little foggy on the number, it was at least 12 though. I'd be amazed if it made the AI smarter but it would make your sim speed bulletproof for when we can play on Huge maps with 12 players. 11 AI with thousands of units is going to be a challenge I would think, unless the 12 player mentioned on their roadmap is just for MP/Humans. 

I've not heard of that particular Zen rumour, cool if it comes true (assuming they are good cores). 

I think the rumors and speculation come from reports like these:

http://wccftech.com/amd-zen-opteron-processor-32-core/

Apparently CERN will be using a 32-core Zen based Opteron processor which turns out to be several smaller cpus connected on die by an interconnect.

Obviously the core count on AMD and Intel cpus for the server market is way higher than consumer variants, but we will just have to wait and see if Zen turns out well and is competitive.

It will be fun if they do higher than 8 cores but 6-8 will be the sweet spot for the average person I think.