Riding in the wet- With narrow tyres you will not need a tread on the tyre. That goes for 28's aswell as the good brand slick tyres are made with a rubber that is "sticky" on the road. Saying that I know my Michelin PR3's are stickier than the Michelin Lithions in the dry- let alone wet. On top of that- the amount of rubber with slicks on the road is minimal so a tread would not be that effective unless you are riding down mountains at high speed with the bike leaned over for the hairpin bends in the wet. But as mentioned- some rubber is stickier than others.

Chipseal poses its own problems with regard to comfort. Run a 23 at 120psi over that stuff and you will be shaken up a bit wheras a 28 at 90psi would absorb some of the vibration. With regard to drag- and I know some will disagree but a 23 at 120 and a 28 at 120 will be about the same amount of rubber- and hence drag- in contact with the road. Only thing is you can run a 28 at lower pressure for the comfort factor.

I had a problem with my first Road bike and that was the tyres. Kenda 26's and rolling speed downhill was not good. There was drag-even at 130psi. I got New handbuilt wheels and Michelin PR2's in 23 and rolling speed went way up. On a particular hill from 30mph to 37. Some of theat may have been the better wheels but I doubt that it was much. Still have those 26 tyres and I put them on my son-in-laws bike. When he wore them out I put on some 23's and he noticed the difference straight away. So Quality tyres do work on the speed side- if not the long life.