I had to laugh just a little. I once bought a K-Line GG-1 that said it was TMCC upgradeable, what a joke! It was upgradeable all right, just put everything needed for TMCC into it and it was upgraded! The one concession was that the pantographs were insulated and had a connection terminal, made it easy to add the antenna.
Lionel used to have TMCC upgradeable stuff that was just a drop-in of the right boards to go from conventional to TMCC, a horse of a totally different color!
Upgrading this to TMCC with the ERR Cruise Commander will certainly improve the low speed performance, and there's room for sound. ERR also has the ERR EP-5/GG-1 RailSounds Commander board for this model.