You are on the right track, however there are two separate issues.

The first issue is that the steady state rated current is the limit at which the motor begins to overheat.

The second issue is that due to the inductance of the coil, it takes some time for the current to build up to any particular value (the time being dependent on the resistance in the circuit, the inductance of the coil and the applied voltage). So, the instant that you first apply voltage to the coil, zero current is flowing. Applying higher voltages gives you more current over a given period of time, and hence more torque, quicker. However, you can't exceed the steady state current limit on average.

To get high torque, use high voltages and a modern chopper motor driver, like the A4988 or DRV8825. Pololu's drivers are cheap, very well engineered and state of the art.

For theory, here is an excellent introduction and probably, more than you ever wanted to know about steppers: http://homepage.cs.uiowa.edu/~jones/step/