Well all this seems very familiar to me. I used to own a group of IT companies and one of them was a storage distributor. We saw large transfers throttling back (to almost nothing) quite regularly and it caused all sorts of support calls to our storage help desk.
My advice is to look at Windoze as the culprit. You've searched Google so you probably already know that there have been raging arguments and bust-ups over this, for example:
Are You Seeing Slow Windows 7 Network Performance?
Unfortunately I can't just prescribe a cure; I can only sympathise. You are not alone!!!!
What Windoze version are you using exactly? Even using a different (older) version may not fix it!!!!!! Good old MS!
Try using a different protocol to test your transfer speeds eg. FTP between Windows and OI. Also see the performance difference if one node or the other initiates the transfer.
Sorry to be so vague but I reckon you can forget about OI being the problem.
Hope that helps.