A lot of aircraft have empty holes for gauges that can be filled/upgraded at later dates if and when you decide to add stuff. My guess would be is that empty hole in Ken's 310R is for the turbo boost gauge that comes in some special 310 aircraft. I have never been able to find a cockpit pic of said aircraft though (Riley Turbostream 310) so I am not sure what actually goes there. As far as 2D panels though I can agree they really have no place in a GA bird since everything is in front of you, but I use 2D panels a lot for more complex aircraft, especially when stuff is out of normal range or at odd angles with my just looking around the cockpit. I don't use Track IR so maybe that is why I still like them.. Just makes things more convenient to me.