I find I need a 10" display to really see everything comfortably. An iPAD mini (7") was just about OK but the iPAD3 straps quite comfortably to my right leg and leaves plenty of room in my P2002JF. Don't need any external GPS to get solid navigation performance.
Despite the good screen on the iPAD it's still not bright enough in full sunshine. Another hazard - at least here in Oz - is that iPADs shut down when too hot. This has happened to me a couple of times and, while it's inconvenient, caused no great drama. But just be aware that these are all consumer grade boxes, and make sure you carry at least the minimum paper charts etc.