I wouldn't think that simple sample mixing would be a CPU-bound bottleneck, unless you combine a lot of effects with the samples. Short of lots of computed effects applied to the samples, I would think your bottleneck might still be bus throughput. Of course, I would suggest you refer to actual testing based on something that is similar to your actual work.
I think the 2.26 8 core mac pro has issues with heavier Hollywood Strings patches because only 1 core can be used per track. That's pretty CPU intensive.