The GP-9 should be a piece of cake to convert, I've done several Williams conversions. I most recently converted a Reading F-7 A-A to MTH PS/2. I have another Reading set that I'm going to do in TMCC so I can run on either system.
For absolute sound quality, Lionel Legacy is still the best, but it's not available for upgrades.
For retrofit command, I have to give the edge to MTH PS/2 conversions, the RailSounds 4 is not quite the quality of the MTH. I keep hoping Lionel will make the RailSounds 5.0 stuff available for sale, I'd love to pick up a couple of sound sets in RS5.
Make sure you're happy with the operation on the layout in conventional first, I ended up taking the command out of that GG-1 because it couldn't handle Fastrack O-36 curves.