Both of those settings might be somewhat problematic with stock EBS volumes. We run everysecond and very frequently see "Asynchronous AOF fsync is taking too long" because EBS can't keep up. The problem is when that happens Redis is blocking connections and exceptions pop up from the clients.

A work around so far is to sync every 60 seconds on the master, and more frequently on the slaves. Another option might be to bump up the IOPS on the volume, but I believe that still isn't available on medium instances (which we are using as well).