Can you try torrenting on system B and see if system A starts noticing packet loss? Definitely not something I noticed, no experience with Overwatch but with other multiplayer games I don't see any when one system is torrenting. I'm also using the MX42BF-UK, most things hardwired. WiFi is off as I'm using a Unifi setup.
I'll see if I can get set up on B and try that at some point.
You have a different router, I have a WHW03 V2. Your one is newer I think.