Yeah, I was guessing that git is actually network-bound or disk-bound often enough that the CPU hit doesn't matter, but I'm not sure. (According to http://cryptopp.com/benchmarks-amd64.html , SHA-1 is 192 MB/s, SHA-256 is 139 MB/s. Faster than either your disk or your network?)
If git holds out for SHA-3 then hopefully SHA-3 will turn out to be faster than SHA-256. There's even a chance that it will turn out to be faster than SHA-1 on modern CPUs!