You should set your STB Video Resolution to Auto and your Graphics (GUI) to 720.
Graphics is for menu's, EPG, Text display etc.
You do not need to use 40% more memory resources to display this information.
Setting your Video Resolution to auto allows the Graphics Processor (Acceleration/ Hardware Decoding) to automatically adjust the Refresh Frame Rate to better match the Stream's encoded Format.
*Note: 1080i is an Interlace format (1st frame - lines 1,3,5,7...etc / 2nd frame - lines 2,4,6,8...etc.)
North American IPTV HD Streams are 1080p (Progressive 1,2,3,4...) and a 30 fps Frame Rate (UK/Euro is 25)
Setting your STB to 1 resolution requires the GPU to Upscale or Pad(re-encode) the Video.
/kens