I'll be thinking of replacing my 5D later in the year, depending on how the £/yen rate goes. I have all the "F4" lenses (17-40, 24-105, 70-200IS, 300IS) and a 50 2.5, because I don't like huge lenses. I'd be pretty confident about the 70-200 and the 50, but some articles I've read raise doubts about the 24-105
Your question was never explicitly stated, but you seem to be asking this:
"If I upgrade, will I get the full expected resolution increase for all of my lenses wide open (except 17-40), or will some of them cause diminishing returns to reduce the improvement to as little as 5%?"
The 5D2 has 28% greater linear resolution than the 5D1. The 24-105 will give you the full 28% improvement across most of the focal length range and most of the image height. The corners at some focal lengths, such as 70mm, are pretty soft, though, so your improvement there might be more like 10%.
The 17-40 stopped down to f/8 will give you the full 28%, even in the corners and even on the wide end. Contrast is pretty low, with some correction for chromatic aberration, distortion, and sharpening, you will be amazed at how much better the images are than on your 5D1.
The 300 f/4 is not as sharp as the other supers, but it's easily sharp enough to take full advantage of the 28% increase.
35mm F1.4 - lots of purple fringing and wider stops, fairly sharp, v good bokeh. Canon should come with an improved version.
A new version would be nice.
70-400 f4 IS - tack sharp in center, not so great elsewhere (fine for portraits perhaps, not so great for landscape),
I can only assume you mean the 70-200 f/4 L IS, but your experience is so different from mine that perhaps 70-400 was not a typo and you are referring to some other lens. The sharpest zoom lens I have ever seen or heard of is the 70-200 f/4 L IS. It has more contrast and resolution in the corners than even prime lenses in its focal length range, and is almost into the quality range of the Canon Supers. It stays on my 5D2 more than any other lens except the 24mm f/1.4. If any telephoto zoom lens were perfect for landscapes, it's this one.
200F2.8MKII (single f.l.) - not quite as good as 135f2.
Agreed.
Both lenses could use an update for improved contrast/flare control and roundness of aperture (they are good, but newer lenses like the 70-200f4IS are a little smoother, for my taste anyway).
Agreed.
The unfortunate thing in my opinion is that most of the lenses out there, even 'L' lenses, don't make the best use of high res sensors (although I still prefer the greater detail from these cameras to the 12mp ones).
I highly disagree. Most of them give the full 28% resolution increase. Some, at certain focal lengths, or in the corners, drop down to 20% resolution increase or so, and a few have extreme corners that are past the point of diminishing returns at 12 MP (let alone 21).
I'm ready for a 50MP 1Ds Mark IV.