We had the same issue at all our boxes.
The solution was to setup other network parameters inside sysctl.conf:
net.core.rmem_max = 16777216
net.core.wmem_max = 16777216
net.ipv4.tcp_rmem = 4096 87380 16777216
net.ipv4.tcp_wmem = 4096 65536 16777216
After changing these values the speed increased from 100Mbit/s to 1Gbit/s.
We discussed this shortly with the support, but they can't explain why the origin values not worked for us.
the original settings have been optimized for compatibility some day in the past. There are a handful of servers around in the internet which do not reply to MWG when MWG runs on those "optimized" TCP settings. In case you ever need to talk to support because you find a host on the internet that simply does not seem to answer through MWG but works fine without or on a default MWG you may have in the lab, please keep the modified settings in mind and let support know they should check if that is the problem :-)