Test performed with a 1Ds on tripod, with mirror lock-up & remote release. IS was turned off on all lenses with IS. Actual 100% image crop of the center. Raw conversion with PSCS - Sharpness, Luminous Smoothing & Reduce Color Noise are all set at zero. No post proecessing except combining all 6 images. Lenes at wide open and also at f5.6 for faster lenses.

The 6 images are from the following lenses and settings: 300f4IS @f4, 300f4IS @f5.6, 300f2.8IS @f2.8, 300f2.8IS @ f5.6, Bigma@300mm @ f5.6 (actually f6.3) & 100-400IS @ f5.6. You can guess which is which (I think the images from the 300f2.8 are very obvious :))