Does that matter (the IAD) if you use the USB output into, say, a BDA-3? Aside from being faster, the BDP-2 should (nearly) equal the BDP-3 via USB (in sound quality), aside possibly from PSU improvements, correct?
IAD affects only AES and BNC digital outputs of the machine. As for USB, Bryston hasn't made it exactly clear what the improvements are in the power supply going from a BDP-2 to a -3.
Power supply in the BDP-1 was a 1Amp but they changed it to 5Amp in the BDP-2. It has a bigger toroid as well and as far as I could see, two shunt-type regulators rather than one to transfer the heat to the bottom panel.
But to be perfectly frank, in my system and our own orchestral works, I wasn't able to spot a difference between a BDP-1 and an IAD-equipped BDP-2 and I had them side by side at one point.
So my conclusion is that you should buy what suits your criteria in terms of system speed, not the audio side of things. BDP-2 is alreaedy as good of a transport as I have heard them and far greater differences can be achieved elsewhere.