Overinflated tires are problematic but I find no difference between 23 vs 25 on flats or rolling ground. There is a slight weight penalty on long climbs, which I can feel after a few hours but nothing that drags me down much...timing is a bit slower for me. I don't use 28s so can't comment on the diff with those. Fast downhills are more confident on 25s and 25s can be inflated slightly lower, which makes them more comfortable. I'm going to use 25s on the Roubaix and 23s on the Tarmac. I currently have the same 25s on both.