I second this.
5Dmk2 has very high MP, hence dense photo sensor buckets on the sensor. If the lense optics is not sharp enough, the image quality will not be good. On high MP sensor, a bit of optics difraction will already be easily captured by the sensor because of the density of the buckets.
This is why 50D which has the most densely populated photo sensor buckets amongst all Canon DSLR lines to date, need L lense to achieve its best performance. Else it will capture less image detail. At f8 and up, it has already been able to start recording lense difraction (hence less sharp).