I tried myself what I had suggested, and sent myself two identical emails via Apple Mail with the above text. The first was text only, and it arrived as a 1.5 KB email. The second was identical but HTML-enabled, and it arrived as a 3.1 KB email, with two body parts: the first was text/plain and was 615 bytes, the second was text/html and was 1349 bytes. When I deleted the html body part (using Mulberry) the remaining email was 1.7 KB. These figures take on very different dimensions if the email is bigger or the sender uses any kind of formatting at all, such as different font sizes, colours, bold etc - to say nothing of embedded GIFs or the like.
For me this missing feature of Mail.app is a deal breaker. But there may be a solution that I'm not aware of.
Of course the best solution of all would be for people to stop using HTML, images or formatting in their emails. Emails were only ever intended to be text only. But I know that battle is well and truly lost.