8.0-RC3 network performance regression

I have several boxes running 8.0-RC3 with pretty dismal network
performance.  I also have some 7.2 boxes with great performance.
Using iperf I did some tests:

server(8.0) <- client (8.0) == 420Mbps
server(7.2) <- client (7.2) == 950Mbps
server(7.2) <- client (8.0) == 920Mbps
server(8.0) <- client (7.2) == 420Mbps

so when the server is 7.2, I have good performance regardless of whether the
client is 8.0 or 7.2.
when the server is 8.0, I have poor performance regardless of whether the
client is 8.0 or 7.2.

Has anyone else noticed this?  Am I missing something simple?

TIA

Elliot