You are way too underconfident. If an intervention is equally likely to raise or lower the score with respect to the control group, without increasing variation, it does nothing.
When you say that the aggregate results "aren't impressive," you imply that they are positive, but if I read table 1 correctly, the aggregate results are often negative.
(by the way, the "active control" group practiced vocab and trivia, which should have no overlap to what's tested by SPM and TONI, which are completely nonverbal)
You're right. I didn't actually locate and compare the unsplit numbers from table 1; I just visually estimated (from the pretty bar chart, Fig 4) the average of the two n-back subgroups, since they're equal-sized. It looks like the n-backers (compared to the trivia/vocab studiers) a non-significantly superior improvement short term, and a non-significantly worse improvement long term.
I'm...
Following up on the 2010 study, Jaeggi and University of Michigan people have run a Single N-back study on 60 or so children.
The abstract is confident and the mainstream coverage unquestioning of the basic claim. But reading it, the data did not seem very solid at all - I will forbear from describing my reservations exactly; I have been accused of being biased against n-backing, however, and I'd appreciate outside opinions, especially from people with expertise in the area.
(Background: Jaeggi 2011 in my DNB FAQ. Don't read it unless you can't render the above requested opinion, since it includes my criticisms.)