It seems, roughly, that SHA256 takes about 40% more time than SHA1. From what I understand, the speed of git is determined most by the speed of the SHA1 implementation (Based on a long thread called 'Linus' sha1 is much faster!'). So roughly, switching would make everything 40% slower.
I think that's a trade-off that they wouldn't be willing to make. However, just playing with the C code of the SHA1 code by the git developers ended up making it nearly twice as fast, so I don't know what the optimal speed difference is against SHA256.
If the numbers stay about the same, I think the git guys wouldn't accept a 40% speed decrease.
(On a side point - subscribing so LWN was worth every penny. How I love to have civil conversations with intelligent people.)