@Marki Yes I'm serious about QoS... you are showing all the classic signs of needing it. At the most basic level, you're sending traffic from a 10GE LAN into a 100Mbps MetroEthernet service with no queuing / shaping. I will respond with specifics when I have time

The kind of speed mismatches you're dealing with almost always wind up causing single-stream TCP throughput problems unless you buffer some of the traffic in the "fast to slow" direction; typically the traffic shaping is done on a router with fairly deep buffers

Don't forget, the problem only shows itself when the flow is 100Mb->10Mb, not the other way round.