Keath,
Do you have Gigabit interfaces on the Sun systems? 
If you have Gigbit try these settings.
NETWORK_BUFFER_SZ 65536
tcp_xmit_hiwat 262144
tcp_recv_hiwat 262144
tcp_max_buf ( should be same size as the speed of the interface 1GB )

Refer to Sun Tuning Parameters Doc
http://docs.sun.com/db/doc/806-7009
 

Your Windows box has a max block size of 65536. I have seen very good 
performance with LTO2 with a block size of 262144 and buffers of 32. 

Something that could be affecting the backup speed, is the ratio of small files 
to Filesystem size. Large amounts of small files slow everything down. 

Blaine Robison
Veritas Certified Professional Data Protection
Sun Certified System Administrator
Sun Certified Network Administrator
Sun Certified Back and Recovery Engineer