Update, for anyone interested.
I finally got around to catching up on my audiobook building. I installed and ran 4 instances of ABB. With Activity Monitor, it appeared I was only using 30% of the CPU, so I installed an additional 4 instances (8 instances total) and was able to build 8 books simultaneously. The activity monitor showed I was using about 65% of the CPU. I have a 6-core 2010 MacPro, with the ABB project files on my system HD (SSD) and the iTunes Library it writes to on a 4-stripe RAID 0 array. I have 24 GB RAM, so the percentage of memory used was minimal. The build is much, much faster than on my old G5 PowerMac. At this rate I will be able to catch up on my audiobook backlog very quickly!
I just can't say enough good things about ABB. It is such a useful program for me as an "audiobookophile." Easily one of the best software purchases (and probably THE best, in terms of value-per-dollar) that I have ever made.