you should be able to play around with the Data Trickling Settings, to see if this helps to resolve your problem. Basically MWG will send only very small chunks of data to the remote end, while it is waiting for the transfer from the Client to complete. If the Web Server expects more data you can probably have success with changing the Data Trickling rates.
If you use HTTPS MWG will decrypt the traffic and you will see the same issue. You can configure MWG to NOT open HTTPS traffic, in this case the data will be moved from the Client to the Web Server without trickling, but you won´t be able to apply filtering on the transferred data.
Our customer verifies that using HTTPS solves the stream problem of the trickling big POST.