MileKing: to directly answer:

Would I still have the same upgrade potential on UA flights? -- no upgrade potential on UA flights with AC SuperElite.

Does it matter if the flights are not AC code-shares? Cannot even upgrade AC codeshares on UA planes.

Would UA flight miles count as AC status miles? Yes plus you would get status on class of service bonuses (and that adds up).

Would my mileage earnings still be 2-3 times UA's overall? My guess would be in the 2+ times range given the types of promos AC had this year. However, I only check promos on the routes I fly. If United had double miles on the routes you fly regularly then you might only have done 1.5 times as well on AC compared to United.

The biggest bonus is the ability to access Red Carpet Clubs for free as an Elite or SuperElite member in AC (provided you are flying on a Star Alliance carrier, including UA, that day).