Thank you, Simon.
Yes, I'll get dual-pivot caliper brakes, because virtually all caliper brakes are dual-pivot; because they, or some of them, are cheap; because I see no reason not to do so; and because I can believe that they're better than single-pivot brakes. What I was wondering about was, (just to look at one brand) are there significant differences among Sora, Tiagra, 105, and Ultegra? (The differences of weight and cosmetics are of little or no interest to me.)
My levers, bar tape and saddle are all fine, I'm happy to say.
(By contrast, I'd very much appreciate totally different gearing. OTOH having what for me is a viciously high lowest gear forces me to "attack" the hillocks of inner Tokyo at some speed, which I suspect is good for me. In the weekends, or when going up real hills, I relax with a bike equipped with smaller chainwheels.)