[CentOS] Slow TCP performance from Win2k8R2 guests under Linux KVM

Would anyone be so kind as to test the performance of TCP streams on 
Windows 2k8R2 guests of KVM virt servers?

virtio-win-1.6.3 was published to address a problem described as "low 
performance" on Win2k8 guests.  Actual performance was not recorded in 
the bug report.

On my KVM servers, Windows 2k8R2 guests are able to send data at less 
than 4Mbps.  I test this by using netcat on Windows and piping a file to 
its input.  The receiving end of the transfer is an un-virtualized 
CentOS system.  Transfer rate is measured by selecting the stream in 
iptraf while it is active.

UDP transfers, such as by CIFS, will send data at around 400Mbps.

TCP transfers from Windows 2012 or Windows 8 are not similarly affected.

The same driver for Win2k8 was published by Red Hat in virtio-win 1.6.3, 
1.6.4, and 1.6.5.

Does anyone see TCP transfer rates from Win2k8R2 that don't look 
ridiculously slow?

https://bugzilla.redhat.com/show_bug.cgi?id=859882
http://rhn.redhat.com/errata/RHBA-2013-0441.html
http://joncraton.org/blog/46/netcat-for-windows