I had a 300D and a 75-300, which I thought was fine at the time.
When I upgraded to the 7D (with a much higher pixel density) I clearly noticed that the 75-300 shots were much softer than the ones I was getting with some of the better lenses I purchased with the 7D.
Yeoer is correct when he says the the 75-300 won't look any worse on the 5D than on the 350D, except around the edges (which in the case of the 75-300, is a significant exception as sharpness drops off sharply at the edges).. But you didn't buy a 5D for mediocre results.
75-300 MTF



