I also did a down and dirty measurement of the R100 paralever and the GS1150 I have in the barn, and the 1150 was 2.5 inches longer. Pivot center to axle center.
I'm saying if the rear is 100mm longer it can get more travel than if its 50mm longer.
I remember somebody saying that 100mm longer was a fair number to aim for, but it had no data backing it up. Just a number compromising additional travel and an over-long wheelbase.