Good suggestion. I have it and it's an excellent lens.

System weight on a 20D of ~4.5lbs versus ~5.25lbs, a difference of 11
ounces or 15%, which is a question of if its worth the cost, since its
a tariff of $100 per ounce.

I think your math is a bit confusing or deceptive here.

His current 70-200 f/2.8 IS with a 2x would become a 140-400mm f/5.6
lens (full range), whereas the 100-400 IS is an f/4.5 - f/5.6, so from
a light-gathering perspective, they're identical at the long end...ie,
both are f/5.6

On the short end, the 100-400 is only a half stop faster (f/4.5 vs
f/5.6).

But that's only until we remember that the 2x can be removed from the
70-200, which makes it once again an f/2.8, which means its at least
1.5 stops better than the 100-400 for the 100mm-200mm range.

Overall, for a meaningful weight savings, I'd be inclined to suggest
giving up on L glass and to consider using the 70-300 IS.  The new
versions are reportedly not as soft as the first generation, its the
same speed as the 100-400, and the system weight drops by ~23oz (33%),
to just under 3lbs, including the 24oz 20D body.

With the DO version of the 70-300, that's roughly $50/ounce, and for
the non-DO version (but still IS), its under $25/ounce.

-hh

I have problems with terminology like this.  To say that a quality TC
"degrades the image" is not very accurate, IMO, and can lead to false
conclusions.  The Canon TCs, the Tamron SPs, and the Kenko Pro 300s do
very little of what I would call "degradation".  Their job, as a TC, is
to magnify the lens' image on the analog focal plane, and they do a good
job of it.  The problems come from users, who don't understand what they
are, and what they are useful for.

The first thing you have to understand is that when you put a 2x
converter on an f/5.6 400mm lens, you are now holding an f/11 800mm
lens, that may need to be stopped down to f/13 or even f/16 to be sharp
enough to warrant this magnification, and may need a doubling of shutter
speed as well, if you're already near the limits of hand-holdability.
If you give the combo what it needs, and the main lens is sharp, you may
get detail unobtainable by the lens itself, from the same distance.
Even if you don't get much more detail, you may get a better sampling of
the detail that is there, as it is spread out over 4x as many pixels,
making the digitization artifacts smaller in a same-size image of the
subject, as compared to the lens with no TC.  If this does not make you
resort to under-exposure, then it can be a gain.  If it forces
under-exposure, then it may not be worth it, as you will have smaller,
but stronger digitization artifacts.

I turned the AF off on my telephotos over a year ago, and I don't regret
it.  I don't use AF with a TC, and I don't use it without a TC, either.
The amount of time it spends lost and hunting is far more than the
amount of time it takes to twist my wrist into focus.  My (technical)
keeper rate has gone up quite a bit since I learned to manual-focus.
Some of the most frustrating moments of my photographic life were AF
gone awry.  Excellent opportunities wasted, because I was foolish enough
to trust AF, and it went the wrong way.

I think TCs are great tools, but you have to be realistic about when
they are useful.  I generally use mine only when the ambient light is
fairly bright, or it is so dark that I am shooting with a high
flash-to-ambient ratio by necessity.  I carry both a 1.4x and a 2x with
me when I am shooting wildlife, and their usage is dictated by the
lighting and the lens.  I use them most often with my Canon 100-400 IS,
which I know by experience is very sharp at close range at 400mm, and
gets softer towards infinity.

TCs are not magical optical devices, but they are useful tools if you
develop the skills and judgement necessary to use or refrain from using
them.

But also realize that where the Canon extenders cannot be used
(usually a conflict between protruding rear lens element and front
extender lens element), the Kenko 300Pro ones may fit.

Bart