I'd forgotten about dual layer. I use a USB loader for my Wii so I don't have to bother with my discs. I remember now that the the Prime trilogy took ages to upload. On the other hand, New Super Mario Bros Wii only took up 0.34 GB of memory.
edit: and i like your idea about including a .wad with the steam version. My idea to Edmund on twitter was much more sinister. I proposed he just anonymously "leak" the .wad online and just let that be the end of it. I'm perfectly fine getting SMB through legit channels though. It just sucks having to wait while everyone else is having fun.