I think the differences will be tiny, and the net effect will probably be insignificant to anybody used to conventional porro binoculars.
I have an SP-roof 8x42 and the 'lack of 3D' compared to my porro binoculars is, to me, hardly noticeable. So it also depends on how much attention one pays to this.
I have an SP roof too , Ultravid, and two porros and generally agree with you except I do notice the 3D of the porros and at times readily see the difference.
This doesn't make me like the roof bin any less. Actually sometimes the 3D effect of the porro looks a little weird to me. I only like smaller sized porros and generally prefer roof bins over porros (not too big as well).
But my original thought was that, if the new Leica is a Perger ( remember they are "upgrading" from an SP roof ultravid) I wonder if a degree of 3D could present itself in the view...that's all. Someone had mentioned Perger will only provide a little more transmission , but I do wonder what other benefits there may be in this design for a birding bin. Like some people see a slight 3D effect in AK bins, would this also be the case in a Perger.
I think I've beat a dead horse on this one. I do hope Leica gives us another clue before the Bird Fair, but they'll probably make us wait.