Some of the pages that I use showposts on require images to be a different size. So while I could lock some of them to be 400 pixels wide, that wouldn’t work for those pages that require a different size.
That could work .. definitely adds some complexity to the process of creating new show posts though. If you have a projected time in mind that doesn’t require adding an update to the functions.php for each show post let me know. I can use this solution for some of the critical pages though. For right now, it’s easier to force a column size and let the image resize with css.