Joomla has indeed copied K2 in many parts, but what they've done can easily be called "overriditis". The chances that you need an ENTIRELY different layout for a single item are probably 0. Design changes are usually per category/section, not for a single item. But even at the off-chance that you do need to do such an override, you might as well use a class name (link that K2 item to a menu to assign a unique class name) to differentiate your CSS rules from the default ones. Even in such scenario, the changes will probably been very few compared to the default design/CSS. If you want an entirely different design & HTML structure, just assign that item to a separate category and use a different set of overrides for it.
