There arent really any "upgrades" to the new designed 4T65e HD that came out in 03-04 from the factory.

The upgrades are left to the trans shops that build these for our cars. Be it clutches, chains, input shafts etc etc. The new OEM trans you got would be the same trans that came in it off the line. Likely it sat on a shelf in a warehouse and was shipped there and installed.

The fwd Cadillacs used the same trans I believe.
IIRC they used the 4T80e and even the newer 6 speed auto in the cadillacs come 08. But both the v8 bonnie gxp and that buick v8 still used the 4 speed.