I think just about all "GM" synths will sound the same, It's almost like they would have to be like that so that GM songs will sound the same on any playback. One thing I have noticed is that adding good quality efxs, like reverbs, makes a big difference to the perceived quality of the sound.
The Roland MT 32 was possibly the first GM module and sounded terrible. But there was an editor by Jeff Kofftinoff that allowed us to get inside and re program the unit, it made it almost passable. My next module I still use is a Korg 05r/w sounds way better, but it's just got better efxs, the base sounds are the same if you turn off the efx.