No hard numbers for you, but FWIW I ran tests about 4 months ago and the performance was /very/ low compared to what is achievable compared to S3 and even normal NAS.
I've been on the list to get into their preview program for a while so I can benchmark it, actually! Part 3 of the blog post is going to include some NFS stuff either way.
When you do, it would be really useful to include the classic fio/bonnie/etc. stuff to break down performance by the type of operation (e.g. file creation / deletion, streaming read/write, random read/write) and block size.
EFS supports NFSv4 so it should avoid being as routinely limited by server round-trip latency as NFSv3 tends to be but it'd be nice to see how well that works in practice.