Just FYI. We have a Peg Perego 30/30 infant seat that we were using in the middle of our 2011 550i.
The base is wide enough that it interferes with the ability to buckle the left and right rear seat belts - it mostly covers the little slots that the buckle receptacles sit in.
I tried pulling on the receptacles to see if they come out of their slots - it didn't seem like they did. Does anyone know if they come out at all? If not, I'd take a look at the bases of the models you are comparing to make sure you don't get one that's too wide.
We've since switched to a convertible car seat since our daughter was outgrowing the seat, anyway.
2011 550i Black Sapphire / Black Dakota / Anthracite
Options: Sport auto/HUD/fold-down seats