The construction of the tire you have - not merely the width - is a critical factor. Also note that the optimal tire pressure varies by tire, width, load weight and road surface. There are a lot of factors, for certain, and I don't think anyone knows enough about them all to say - factor a is responsible for 26%, and factor b is responsible for 19% (and so forth), and many bikes' handling can change appreciably for better or worse between running 21mm tires and 28mm tires, so there's no blanket statement of "use this size" or "use this brand", but I'm more than satisfied with what I have on the old gal now.

A couple years ago, when I was running the same bicycle, with the same hubs and 21mm tubulars, I did not have the experience of having to brake multiple time coasting down hill to keep from overtaking other coasting riders.