But how does the gas engine do in sound pressure test? That is also controlled in major events and in my experience the gas engines are much louder than the YS or electric. I may be wrong about this...

By the way, build up surfaces tend to be lighter than full composite surfaces. The fuselage may be the opposite case - full composite ones can be lighter than balsa/ply structures.

I sort of agree that the weight limit could be lifted a bit to make it easier to get an ARF 2m pattern kit under the limit. For example, my Spark Evo 2 is some 200 g overweight at the moment which is mainly caused by the motor which weighs about 670 g with prop mount and mounting hardware. This motor works just fine and delivers more than enough thrust and costs only about $150. It will be replaced with a Hacker C54 which is 170 g lighter (and considerably more $$$) soon... However, making the 5 kg (or 5050 g) limit is not the only reason to change: A lighter plane will fly better and the motor is higher quality and will last for hundreds of flights. There may be other bonuses as well like easier throttle management and better braking etc. It will also cool directly from the built in air scoop of the Spark without carving the spinner cone full of holes.