"I'm no expert on astrophotogrphy, but it seems I've read the 24/1.4 at iso 3200 is good."
It is very good indeed out to the corners by about f2.8 on a 5D series body. Photographing stars with fixed tripod mounted cameras is definitely an area where FF is much better than crop. I've done this many times using my 7D and 5D2, the FF wins.
Here is a 24L sample at ISO 3200 f2.8 on a 5D2, particularly note the star images top right corner:
