My experience is quite similar to what is described in the first article -- no coffee leads to better concentration for me. The caffeine 'boost' I was talking about reduces my concentration but makes me more inclined to action -- I found it useful for breaking through procrastination periods. The effect of Red Bull on me is similar but more pronounced.

The effect seems to be physical, but I don't rule out placebo (and frankly, it's fine with me either way.)