A speed drop with multiple copies is not typical for Solaris.
The problem may also be at the Win7 side (Network card, copy tools etc)
Same happen with another host running Win2k8 R2..
I am starting to look more toward ESX or something.. but all this dosen't make much sense to me as it can handle 700Mbps+ on single transfer.. I might try to install OI directly on the host without ESX..
What's also interesting is it seem to go down to 100Mbps...