The basic premise of the gMotor engine is having max 1000polys per GMT group, then it doesn't matter how many GMTs your track contains as long as your LODs are reasonable of course.
I have seen major performance increases in certain tracks upon merging materials. IIRC I edited Fuji 2007 for the RDLMS, and I gained 20FPS (from 50-70) through material optimisation. This included some texture resizing also I believe, but nothing that would gain 20FPS. However, other tracks I have found no performance gains through material optimisation.