The problem is hyperthreading technology, don't think AMD can compete with Intel. And Arma with it's millions scripts executing at the same time is just what the doctor ordered. What we observe is scripts queuing up, and the more the longer it runs while processor never goes above 30% and mem over 2 gig
I didn't think it mattered since the amount of cores + threads that Arma could use were limited.
I used to buy the higher clock speed the better theory but it's not how it worked out for us, obviously a cheaper E3 would have saved a shit stack of money over the E5, however I have never had any issue with Arma running cores/threads @ 2 to 2.8Ghz.
What the extra cores have allowed, is for multiple instances to be run (many more) and other stuff on the box for BF3 etc (we use vmware esxi) looking at the stats over the last few days only 15% of the CPU is being used 64GB of memory, Disk latency is 0ms and spikes to 1ms during backups, that's with around 85,000KBps usage and network sits at 3Mbps constantly.
We always run out of memory before we do CPU, disk used to be an issue but with SAS disks and raid 10 it hasn't been a problem for a while. Next server we buy in eight months time I will probably save money on the CPUs and go with lower end units as 85% of my investment sits there idle the majority of the time.