Joost:
I also think that a Porsche is more seen as a daily driver, and customers will not accept the cost of reparation of a CF tub. McLaren is more seen as a supercar with the associated cost. They can afford to charge sky-high prices for reparation, where Porsche can not (for the 911 range).
Even more than potential repair issues, the CF tubs I am aware of all have a fairly wide door sill that one has to slide over to reach your seats, part of maintaining the strength needed. OK for a supercar-style experience but not in the DD use of a 911. McLaren made a big deal about reducing the height of this sill in their 570 series, but it is still much more prominent than in a normal car that does not use CF.
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2017 Range Rover Sport S/C, 2009 Porsche 911S