The problem is that you have a finite amount of power from the motor. in a low gear, there's plenty of torque, but each time you change up a gear, the torque at the wheel goes down. as your speed increases, the resistance to motion increases as well until the torque from the motor is giving the same force as the resistance to motion, which happens on the flat at about 22 to 25 mph depending on how hard you can pedal. If you have a following wind or a downhill, you can go a bit faster. At that speed, changing to a higher gear doesn't increase you speed because your torque will go down, so you'll slow down instead of speed up.
To push a MTB along at 30 mph, you need about 1000w. I would guess that a Bosch motor can sustain about 500w of output power. A fairly fit cyclist can sustain about 200w, so the 700w would give about 25 mph as a sustained speed on the flat with no wind, but you would be pedalling quite hard for that.
In summary, you need gearing that allows you to pedal at a maximum of about 30 mph to get the best speed out of the system. Higher gearing will only help if you have lots of downhills. Take your bike to a good downhill. If you can pedal at 30 mph, your gearing is fine.