So I think what I really want is an add-on or extension (I use both Safari and Firefox) that selectively changes the user agent to Mobile Safari.

Most sites are pretty good with providing iPhone compatible content, which is magnitudes smaller than the desktop version.

For example the techrunch page from my previous comment comes in at 33 requests and 122 KB when I set Safari to "Mobile Safari iOS 4.3.3 iPhone".

If only Safari on the iPad supported extensions :-/