Is there a way to turn on window scaling in 4.1?
Well, there are a lot to consider when you say transfer speed.
1. Separate the management, VC, heartbeat networks and vmotion networks
2. Separate the VM Network to a separate vswitch and try to pitch in a load balancing
3. Try to check with tcp offload as here http://kb.vmware.com/selfservice/microsites/search.do?language=en_US&cmd=displayKC&externalId=2008925
4. The transmission speed is again based on the levels of nic speed and routes, it will not change much to a higher number if you have 5.1
5. Check the possiblity of 10G card which is supported vastly in 5.1