I don’t see any reason anyone should have to “sell” a client on CSS. If a client is well informed chances are they will want their site to be CSS based. If they aren’t informed then there is no reason they should be bothered by the developer using best practices, and it is considered best practice to use CSS by the overwhelming majority of the web development community.
In regards to speed… given that Tables require twice as many tags as CSS does it is impossible that a competent designer will be able to create both at the same speed. If you aren’t hand coding you can’t go very far with systems like Drupal which are template based and therefore won’t display properly in Wysiwyg web tools. But really, that’s not even scratching the surface. Once you start styling the page CSS leaves tables in the dust. Side by side there is no way a table based themer could keep up with a competent CSS based themer. With absolute control over the styling of the entire site all delegated to one style.css page you can do a lot more in the same amount of time. If development time is not a good argument for a client I don’t know what is (after all time is money in this business).
And again there is the issue of what CSS is capable of. Table based layouts can’t achieve the same effects. They are much more limited. So what you offer to the user is also more limited… difficult to quantify, but easy notice, even for a layman.
Then of course there are the performance issues that you outlined. In my opinion those are more than sufficient arguments to side in the favor of CSS, but for me I don’t even need to get into that to tip the balance. I was convinced just by the fact that CSS is faster and easier to maintain. After all, laziness is the mother of conventions.