When the shower current is closed using the OEM track position, the curtain definitely intrudes a bit into the space occupied by someone on the toilet. So if you do move the track, plan on keeping the curtain open whenever the shower is not in use.
Otherwise, it seems like it would work fine. You're not going to save any time by doing both mods simultaneously, so you could just line the walls with plastic first, and see how that works. If you're satisfied with the additional space, you don't have to move the track.
Dave