because I am not too familiar with AA's program, i am leaving the issues of mileage accrual to others. Suffice it to say that non-airline mileage accrual on Asiamiles is mostly limited to offers in Hong Kong, and very few offers in other Asian countries.

If a lot of your travel is on CX, then the MPC is worth it, no doubt. However, anecdotal evience suggests that AA elites get treated better on CX than MPC elites on AA. (bit a of a David and Goliath going on there). Upgrades on CX are common for top tier members on full flights, where the pecking order decides who gets to fly up front. Every airline gives priority to their members.

Facit: Switching to Asiamiles as a mileage program is probably pointless unless you live in Asia. Joinging Asiamiles (and not MPC) will not yield any benefits whatsoever. The elite program of CX is MPC, not Asiamiles.
Switching to MPC is a good choice if you travel a lot on Cathay and value upgrades highly (which seems to be your sole concern anyway )