After a little investigation I didn't find any recent information on how well / linearly 389 scales from a CPU perspective. I also realize this is a more complicated topic with many factors which actually play into it. 

Throwing the basic question out there: Does 389 scale fairly linearly as the number of CPUs are increased? Is there a point where it drops off?

Where am I going with this?
We are faced with either adding more CPUs to the existing servers or adding more instances or a combination of the two. The current servers have 10 CPU with the entire database fitting in RAM but, there is a regular flow of writes.  Sometimes somewhat heavy thanks to batch updates. Gut feeling tells me to have more servers than a few huge servers largely because of the writes/updates and lock contention. Needing to balance the server sprawl as well.

Thank you for your insight -
Ben