This is not a very scientific comparison of the EF 75-300 f/4.5-5.6 iii and EF 70-300 DO Canon lenses on both the Canon 400D and 5D.
Unfortunately the light levels were changing between shots.
On conclusion, looking at resolution of 400D being double that for the 5D on the same size area of the object being viewed, only give a marginal improvement. It seems to me get the most out of the 400D, only lenses with very good optics can be used.