Go into Auto Resolution in the setup menu and change "show info on screen" to no, that should sort the writing in the corner out. In the AV settings have you got it set to "just scale" both times??
Some of the content on the HD channels is not native HD, so sometimes programmes will not be the size of the screen, there will be bands down the left and right side's, about the only thing you can do here is use the "ZOOM FUNCTION" of your television to make the picture fill the screen, not much else you can do.
Also mate if you are used to a 500 a 800 will be slower thats just enigma2 i'm afraid.
Glad to here everything is going ok.