If I remember my Physics classes correctly, more mass just means you will need more force to get it going and to stop it. You either need to change the acceleration (gravity) or increase the time that goes by (Velocity = Acceleration * Time). For instance, a ping pong ball and a bowling ball would roll the same velocity because acceleration (gravity) is the same on both. Obviously there are other things that effect this in real life, like friction and air resistance, but I doubt Blender takes that into account.
Now with a ramp, the shallower slope will have less acceleration than a steeper one, in the direction that the ball is going, as you have already done. Since changing the ramp is not a choice and you want to do it in less time, the only real option would be to increase acceleration, gravity. Which I do not know how to do in the Blender GE.