I honestly don’t think many clients care about this. And I think it could backfire.
I think that this… like many things, would be dependent on how it’s framed and presented. One thing I will DEFINITELY avoid is suggesting sites to check it against. I would never say “Compare our site to x.” But… if they want to use it in that fashion… that’s cool.
I think it would be a very helpful tool, as for the most part, file sizes have a direct correlation to download speed. Is that the whole equation? No… but it’s a huge part of it. And I want to build a brand around insane speed and lightweight design, without sacrificing eye-candy one iota.