Hmm. I've (independently from the original poster) got the 8k sizes:
with the network set up for jumbo frames (MTU=9000) the nfs server then
chunks out network packets that get through the switches with minimal
overhead. Making this change from the default (1k, MTU=1500) made a
huge throughput difference at the time we implemented it: which was a
number of years ago, so certainly the world has changed since then.
How do 1MB sized nfs chunks interact with the networking?