This blog post is worth reading: http://calnewport.com/blog/2011/12/02/is-talent-underrated-making-sense-of-a-recent-attack-on-practice/
But in refuting a basic error, it commits another one. The article it is working to refute states that "talent" (a word liberally used to refer to working memory) is important to success. I wont' go into the extra complexities here, but the article basically says that you should focus on the stuff you can control - and let the working memory worry about itself. And that's the failure.
I have read nothing to indicate that working memory is genetically determined or even genetically predisposed. I've read several sources which strongly suggest that working memory can be expanded - by direct and indirect exercise. I've certainly tested better in working memory tests since practicing Dual-N-Back. I've increased both my number and my wordspan with practice.
I will continue to experiment, and report back with updates periodically as I progress.