They benefit only from XG instruments, because XG is an extension to the GM sound set, in sounds as well as in other areas.
Think upside down, if XG midi files could make a difference in all intruments, there would't be a market for Yamaha XG modules and keyboards.
Because (as Yamaha stated) the XG is much more than an extended sound set, sometimes XG files will play very very different on GM instruments, to the point they become unusable.
Theodore