I think you'll find some variance in the recommended PSI between varying 28mm tires. What tires do you have? What tires did you replace?

Failing to inflate your tire to a minimum PSI can leave you more prone to flatting (pinch flats) and possibly rim damage. So you can only lower your pressure so much.

A heavier rider would require higher PSI than a lighter rider, all other factors remaining constant.

There's a good possibility that a larger tire (such as a 30-32mm) at a lower PSI would be just as fast...but more comfy...than your 28mm choice. Lots of opinions on this, though, and the variances in tire construction and materials used will play a role here.

All opinions, of course...