You've got a few points in here so I'll try to pick a few out..
The speed issues - best queried in the broadband forum but you need to establish if this is a speed issue or a WiFi issue. VM only guarantee speeds over hard-wired Ethernet connections, not wireless.
The 360 mini-box doesn't have a hard drive by design, it's reliant on the main box for all hard drive functionality. The master box needs to be set to fast start or active start to allow the mini to avoid issues with recordings.
Inability to play anything without buffering - what are you trying to play? (Recordings or VoD?) And is this on the main box or the mini?
How are your 2 boxes connected to your home-hub.. wireless or Ethernet?
You won't want to hear this, but once you've migrated to 360, you can't go back to the TiVo platform. Had you swapped the TiVo for a V6 before migrating, you'd have kept both V6 boxes and had 2 hard drives.
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