I don't know...I would not spend too much of my time hunting around at this issue. The main thing is the isolation designed in for wifi and bluetooth. The more isolation you have, the greater the data-rates and distance you obtain. The limits are going to be imposed by the design of the iPad...and then beyond that, it's a matter of what you are actually trying to transmit over the two antennas..so performance could indeed be fine in one situation and not in another. In the case of my presentation, I've sending video information (mostly static, but there are changes happening on screen if you use a cursor and change slides rapidly) between the phone and iPad, and all of that depends on what's on the slides and how fast I move though the pages, and also what's being seen on the wifi side. There will be inference between the two. Electromagnetically speaking, your environment could be very different than someone elses. I know that in my office I sometimes have trouble getting my BT speaker to connect to my ipad or even my BT keyboard. If I go downstairs, a bit further from my router, I can get them to connect and then come back upstairs and they remain connected.
IME, BT is much more rarely used than wifi, and wifi is much faster. I've used BT for a wireless link between a gps receiver and my ipad in my car, and it works great. But there is usually no wifi signal to contend with in the car....and BT info for GPS is minimal in terms of data rate.....now that I have 4G, the issue is probably even less since they are more separated in frequency.
There are ways to increase isolation beyond just where you locate the antennas in the device. You can use polarization control, or include lossy materials, or even employ artificial magnetic conductors. The latter works extremely well, but you need space inside the device to put those AMCs. I have no idea if Apple is using such tech, but I doubt it. As you know, iPad are devices where space is at a premium. This has to have an impact on the amount of isolation that can be achieved and the the cost to improve it significantly might be deemed of little value, given that BT is rarely used. Based on what you say Apple has told you, it sounds as if they have made engineering decisions and you are just unfortunate enough to be hitting up upon them. It seems like the conference pad app bumps into them as well. As secretive as Apple is, it is no wonder that this is a bit of a mystery. But I've seen these trade offs made in laptops and they are fundamentally the same ones that need to be made for an iPad.
Boy...there must be a better way to type a long message. I can only see about 1.5 inches of what I'm typing, so getting the typos out is a bit of a bother. Anyway, good luck hunting!