I just learned that sha512 is faster than sha256 (on 64bit platforms, see benchmark below). attic uses sha256 at some places, could/should it be replaced by sha512?

Attic uses hmac-sha256. I did similar measurements for hmac-sha256 and -sha512 and the speed was as shown below.

$ python3.2 -m timeit -r 10 -s 'from hashlib import new; h = new("sha256"); x=b"foobar"*1000000' 'h.update(x)'
10 loops, best of 10: 29.2 msec per loop

$ python3.2 -m timeit -r 10 -s 'from hashlib import new; h = new("sha512"); x=b"foobar"*1000000' 'h.update(x)'
100 loops, best of 10: 18.6 msec per loop