==> There are two problems with relays:
1) configuring the relay on 6to4 router; anycast address helps a bit there.
2) v6 nodes finding the closest relay advertising 2002::/16. This is not
addressed here, and I think is a major problem: there is zero guarantee
about how unoptimal the return path is (you can make some assumption about
the forward path, to the 6to4 relay close to you)!
It strikes me that no tunneled infrastructure (automatic or manual
setup) is likely to grow dense enough to provide really good
performance, at least not without becoming unmanageable in its
complexity. I have always regarded that as a fundamental tradeoff
when using tunnels for anything, not just IPv6.