My system is two Garmin G3X Touch screens and a GTN650, but there should be some overlap between my experience and your question...
I've never had good luck with keeping the iPads and iPhones cool in the RVs. Granted, they were never mounted vertically, but still...
There's also the question of how you're going to use all your physical screens, whether panel mount or iPad. In my setup, I've got four half screens plus two skinny engine screens. Two of the half screens are flight instruments, one on each side, and the other two half screens are towards the center of the plane, readable from either seat. The GTN is used for flight plan editing, approach selection, and for destination waypoint information.
Two points: It's not just what displays you have in the plane, it's how you use them -- what information gets displayed where in which phases of flight. Part of that analysis is separating which button pushes do useful things directly, and which button pushes take you from screen to screen. The idea is that button pushes that take you from screen to screen are "overhead," and, ideally, should be minimized. If your trial screen allocation has you doing lots of screen swapping, that's a poor design.
The second point is that people in general do a poor job of figuring out what they're doing and how well they're doing it. The prime example comes from Apple Computer in the pre-Macintosh days when there was tremendous debate on how fast the mouse was compared to arrow keys. They ran timed tests, and everybody swore that arrow keys were faster. The stopwatch disagreed...
So here's the challenge, and it's a tough one. Figure out how many screens you'll want for each phase of flight, and if you sometimes don't have enough screens in the cockpit, which ones will time-share the displays or just get dropped. (This is a really tough task, and I have not done it myself.) Then figure out how you will distribute the useful information in the cockpit, with and without the iPad.
My choice? Put it all on the panel. (This also means that I don't have to come up with an iPad mount.) I don't want a device that might be hard to read in direct sunlight and that could overheat. And I can have two half screens on the panel vs only one (possibly) on the iPad.
It's also worth noting that pilot adapt well to all kinds of circumstances. There are homebuilts out there that the owners love but, objectively, handle poorly. There are user interfaces out there that, objectively, are terrible, but pilots adapt and love them.
And, yes, I have an iPad with foreflight and almost never use it. I do use foreflight on the iPhone but for flight planning, but not for flight plans.
Good luck!
