Pg_upgrade performance

I've been having a look at this guy, trying to get a handle on how much 
down time it will save.

As a quick check, I tried upgrading a cluster with a 1 non default db 
containing a scale 100 pgbench schema:

- pg_upgrade :         57 s
- pgdump/pg_restore : 154 s

So, a reasonable saving all up - but I guess still a sizable chunk of 
downtime in the case of a big database to copy the user relation files.

I notice there is a "link" option that would be quicker I guess - would 
it make sense to have a "move" option too? (perhaps with pg_upgrade 
writing an "un-move" script to move them back just in case).

Regards

Mark