The router may be top notch, but it's still a router with a part time job as fileserver, and not a nas. Normally a router doesn't get high throughput as fileserver. On the other hand, this box has USB3 ports, which suggests that it should be able to saturate an USB2 port (at 25 MB/sec). But of course it can also be marketing.
Unfortunately I cannot find any reviews where the box' performance as fileserver is tested.
I can only reach some miserable 15.5Mbps Upload and 11Mbps Download
Do you mean Mbit/sec or Mbyte/sec? In case of Mbyte/sec that is not bad, actually. That boils down to 150 Mbit upload, which might be the max of that homeplug set.
To really test the fileserver performance, you should connect your PC by a gigabit capable utp cable directly to the router.