I love that my kids have grown up in the back of a 911!
Based on my experience I think the upper tether mount adds a lot to the overall stability of the seat. I don’t want to argue the crash protection merits of an upper tether point, but the day-to-day stability of the seat (in either 5-point mode or just used as a booster) is greatly increased with the tether. The reason for this is that even though the booster fits PERFECTLY in the seat pocket area, the (Porsche) car seat design is very deep b/c of the transmission. The seating is deep and the seatbelt anchor attachment points are relatively high in relation to the seat bottom (normal cars have a pretty flat seat with belts right there, the Porsche belts start just a little higher and forward).
So in the Porsche, the seat fits perfectly and the seat belts thread through nicely but the triangulated relationship of the belt is altered causing most any booster seat to tip forward. (Sorry for the long-winded detail – see pictures)
When I was first installing the seats, I knew everything was locked down and secure, but given the forward and high location of the seat belt anchoring bolts it still felt tipsy. The upper tether mount completely solves this.
Upper tether mount
