The handluggage weight limit is actually 15kg in the US and not enforced. Lots of people never check luggage, carry it all on. CX allows an extra 15kg for Sapphire in their own plan (MPC), they don't have to extend that to AA Plat but I think that they will. At CX only supervisors are authorized to make decisions, so be sure to ask for a supervisor if you need something special done.