A Norcal Leopard will have a different shape in the head when compared to stock. There will also be different squish measurements. They should be easy to catch if you know what your looking for.
But the gains are not significant for most drivers. It is easy to be fooled in thinking a guy/gal has a hot motor if they come off the corners faster than another driver. A chassis that is rolling free will get the power down much qucker and carry much more momentum than a chassis/driver that is not. The difference can easily be 4 to 5 kart lengths. This will translate into 3 or 4 mph down the straight. I hear it all the time, “Its the motor” when in fact its the chassis/driver.