In addition to power concerns of many fast cores on one chip, server chips (the multicore champions) tend to be larger dies, and place higher value on MTBF. I believe both of these factors work against super high frequency server offerings.
Right, and that makes sense. Larger dies are primarily a cost issue; they reduce yield, which increases cost per functional die. More development effort spent on MTBF is a resource issue; work spent improving MTBF is work not spent improving operating frequency, but with enough resources (money) it doesn't have to be a problem.
I could be wrong on this, but I believe IBM's AIX line is not cheap, if you see where I'm going with this.
You're definitely going in the right direction; Power5/6/7 hardware is very pricey. I'm amazed that in this day and age, AIX still has a place in many companies, and slightly perturbed that I have to maintain it. I'd much rather have the applications running under RHEL.
In addition to power concerns of many fast cores on one chip, server chips (the multicore champions) tend to be larger dies, and place higher value on MTBF. I believe both of these factors work against super high frequency server offerings.