Road bike tires are generally round in profile. I know there is a slight variation in there, but it's not much. The only way to get a 23mm tire that has a lower profile than another 23mm tire is to buy a tire that is actually smaller than its stated size. For example, the Continental Ultra 2000 or Grand Prix are actually something like 21mm when they're labelled 23mm. But in that case, you're really just buying a 21mm tire.