I've built tables with 3Nm (424 oz/in) that get 2500mm/s/s (~100 in/s/s) no problem - in fact that's backed off so I don't jerk the hell out of the things. That gantry looks really light - you MUST be able to get a LOT more accel out of that puppy. What's your power supply, drivers, motors specs, drive ratio?
Ian