BIG BAD WOLF: We're talking about text here - page size isn't significant here. You won't note the difference between a few letters more. Rendering the page however may show a difference - and that's what you can surpress with display: none.

As for the size of the the CSS file, two things. First, having ids and/or classes for all elements doesn't mean the CSS file should have an entry for each id or class. Second, a CSS file is rather static - the CSS file typically doesn't change between page requests. Which means the browser only has to check the CSS file hasn't changed, and doesn't have to download it.

As for the server side calculations needed to generate a page, I have no idea how much is calculated on the fly, and how much is pre-calculated.