Will pg_repack improve this query performance?

I believe this query is well optimized, but it's slow if the all the blocks
aren't already in memory.

Here's example explain output. You can see it takes over 7 seconds to run
when it needs to hit the disk, and almost all of it is related to checking
if the user has "messages."

  http://explain.depesz.com/s/BLT

On a second run, it's extremely fast (< 50ms). So I'm thinking it's a lack
of clustering on the "Index Cond: (to_id = users.user_id)" that's the
culprit.

I'm afraid of using CLUSTER due to the exclusive lock, but I found
pg_repack while researching:
http://reorg.github.io/pg_repack/

Does it seem likely that doing an --order-by on the to_id column would have
a significant impact in this case? pg_repack seems pretty stable and safe
at this point?

I am going to try and test this in a dev environment first but wanted
feedback if this seemed like a good direction?

Thanks.


-- 
Best,
AH