I was thinking of getting a 7a for graphene, but I think I'll wait for the 8 to get graphene support and spring for that.
From what I can tell graphene really is a much more secure OS from an encryption and tracking perspective. Its multi-profile configuration and Google play sandboxing alone would be worth it.
OTOH, I have used it twice in two models of car, and in both instances it would stop working until I unplugged and replugged the phone. I just keep the phone visible in the cup holder now.