As others note, you would get access to the FCL as a Diamond- but if you credited the flights to AA, hit EXP you'd have that benefit as well.

The main benefits I can see are seat guarantee and possibly better service in case of IROP. CX treat OW Emeralds fairly well though so you'd just need to decide whether those two things are worth giving up domestic benefits and the elite RDM bonuses (which can be huge for TPAC flights).

Nothing special onboard apart from first choice of meal.