I just did a test comparing the Zeiss ZE 50/1.4, the Canon 50/1.4 and the 24-105 @50mm all shot on a 5D Mark II. The Zoom then is a little bit softer, but not by too much. Either the primes slightly barrel or there is an odd pin-cushion distortion with the 24-105 at 50mm. Maybe someone else has a clue about that.
Regarding resolving power, at normal zoom its really not too far behind the primes. Not ideal but definitely sharp. For 16x20 i think you'd be hard-pressed to see a difference