Can you post your ini file for us to look at?
I am pretty sure there is a way to write the list so you can use the GM sounds in performance mode, instead of looking at it the other way around and trying to use the extra sounds while in GM Mode.

For my Roland it looks like this.

[Juno-G]
1= Acoustic Grand
1,121,0= Acoustic Grand
1,121,1= Piano 1w
1,121,2= Euro Pf
2= Bright Acoustic Piano
2,121,0= Bright Acoustic Piano
2,121,1= Piano2w
3= Electric Grand Piano
3,121,0= Electric Grand Piano
3,121,1= Piano 3w
4= Honky Tonk Piano
4,121,0= Honky Tonk Piano
4,121,1= Honky Tonk2

The default is used and the others are available as options in my patch map.
After this first section I go on to list the other 1000+ patches in order of Banks and Collections .. I even created commented out sections for the various SRX cards that existed at the time (including the known MSB LSB info) so if someone had one I didn't, they could just add their new patches there.

Or, maybe that synth can't do this. Just a thought on how I did it, with a Roland, though the Roland I use isn't as old as yours (but not really newer either).

I do not work here, but the benefits are still awesome
Make your sound your own!