I didn't go in search of any more information than what was given, but the BeSafe iZi didn't mention the type of collision in which it rotates. I mean, it's called Side Impact Rotation, so does it only rotate in side impacts or all impacts? Does it rotate toward the center on a far side impact or does it rotate toward the near door on a far side impact?
I actually have the iziUp Fix seat sitting in my cupboard. The "side impact rotation" feature only works when isofix is used. And I think it will rotate somewhat in any crash that happens at an angle, because the design is that there is kind of a "base" that attaches to the isofix anchors, and that the rest of the seat [seat cushion included] is attached to the base closer to the end where child's knees would be but it also swivels a bit [I haven't tried how much it would turn cause i don't want to break it obviously]. I think the idea is that once struck from the side the base would more or less remain in the position it was [relative to car's seat/isofix anchors] because of the rigid isofix attachment, but the rest of the seat would turn to the side somewhat because of inertia, thus putting more of side wing stuff in the direction that the side impact came from [sorry, English is not my first language :| I hope what I wrote makes sense].
The iziUp Fix seat is also very heavy and has extremely deep head side wings, in fact so deep that in my smallish car they made my DD lean forward to see out of the window, so we're not using it now.
An acquaintance of mine has a Britax booster with the XPad thing, and she said that that the seat belt tends to end up in twists within the pad; also I've heard that the belt doesn't retract nicely through the pad in all cars. I've been wondering if the pad could also negatively impact features that seat belts might have, pre-tensioners and load limiters.
Added:
Honestly the BeSafes SIR mechanism bothers me, on one hand it kinda has more potential working because it seems like it wouldn't be affected as much by the frontal component of a side crash unlike those side impact cushions that every other booster in Europe now seems to have, on the other hand I've never seen another booster with a similar feature and I feel like it could also backfire? This is more like a "gut feeling" of mine, not based on facts. I.e. what does the rotation do to the lap belt placement? Doesn't it actually move the child closer to impact? What if it's a multiple event crash? Etc.
The XPad I can unfortunately see causing issues i.e. the above mentioned twisted belt or belt not retracting. I tried my DD in the Britax with XPad seat, she was only wearing thin summer clothing and it took like 10 seconds until "Mooooom, this thing is bothering me!" Granted she could get used to it, or wear a cardigan or something....