Look very closely at the R26 and you'll find the limb pockets have been modified and are indeed different from their other bows. An R20 will in fact shoot slightly faster than a R29 but the 1/2" increase in power stroke is just not significant.
IMO, they don't need more speed at all to once again set the bar higher. It's not a matter of flatter arrow trajectory but a matter of accurately predicting that arrow trajectory so that the arrow can be delivered more precisely to the target with very little if any additional effort by the shooter.
If you are concerned about arrow penetration at longer distances, and that's why you want more speed, then a skinny arrow can easily address that concern.