Thanks for the updates @NJacobe it sounds like we've collectively ruled out everything we can, time for a case, let us know what it turns out to be. I agree it's especially odd the uplink is being held to 20Mbps, almost sounds like the typical asymmetrical traffic shaping many ISPs use, but when you bypass the MX it's gone, yet the MX had no traffic shaping rules in place. One last thing - we didn't overlook a group policy did we? The MX firewall and traffic shaping page might not have any settings, also make sure you're client isn't being tied to a GP that might be enforcing limits.
... View more