I'm from Europe (I imported the DGL-5500 as it's unavailable here) and also I am seeing quite a big difference in Streamboost on and off.
For testing, the only cable connected to the router is my PC/
With streamboost: 120.8 Mbps down , 13.1 Mbps up
Without streamboost: 151.1 Mbps down, 14.4 Mbps up
I did not checkbox "enable auto bandwidth estimation"
Download speed is set to 150.00 Mbps and Upload speed is set to 15.00 Mbps