Can an amplifier deliver a higher power for a short burst as opposed to continuously using the same circuit
The average power reduces as you lower the duty cycle and this usually means less heating in the power transistors and power supply to the amplifier. So, if full duty consumes 418 watts, 10% duty consumes 41.8 watts in the load.

The current will still be 7.5 amps when driving the load and will be zero amps when not driving the load. The RMS current can be calculated by the average power of 41.8 watts into 7.5 ohms thus, RMS current is 

\$\sqrt{\dfrac{41.8}{7.5}}\$ = 2.36 amps. 

Note that the average current may also be a useful quantity.

More than likely you will be able to use fewer transistors in the output stage.