Agreed that the lens should be performing equal or better to previous bodies for a print of the same size. But, when Squidy mentions zooming in and not seeing clear details, then that's him looking for details in the extra resolution the 5Ds offers, and wondering how to get those details. That's the question he's asking. Would the new lens improve sharpness/detail at the pixel level... and that's all my above answers were about.

If you aren't concerned about the extra detail at the pixel level that the 5Ds offers, then you don't need a 5Ds. The whole point of the 5Ds is to get those extra details. If you're fine with the pixel level details being soft because it will print the same as your 5D3, then stick with the 5D3.

Which bring the argument from 'same photo, printed same size, looks the same' to 'I want more detail when pixel peeping'.

I also agree that camera shake isn't a good reason to not get a 5Ds. It will affect the sharpness at the 100%. Faster shutter speeds or better technique or a tripod are more important to the 5Ds than a 5D3. Then again, your old technique may have been good enough. It's only a concern when your technique/shutter speed was only marginally okay on the lower density sensors. But, if you really want those details, you'll bump the shutter speed, or invest in a good tripod, or both.

AFMA is a good point too. I adjusted a few lenses on my 7D2, and it made quite a difference, even if the change was only +1 or +2.