I use GR 5 configured as a synth all the time, so that I can use both a Behringer FCB 1010 midi pedal board and the controls on my Axiom Pro 61 controller to control GR 5 knobs and wah wah and what not.
I would agree with trying to gauge effect of splitting those simple instrument tracks out, just to see if that helps.
I will try to set up a test of all of what you are trying to do, and will psot back with my results,
Bob Bone