Could you only drill the holes and do nothing else to the box to achieve a louder sound with no down sides?
yup. unless someone has a before and after dyno result for a purely smoothed box i really doubt that makes any difference. I think the raised bits are there to serve a purpose, i cant be bothered to explain why but i;m like an old man on this, stuck in my beliefs rather... Mine is just drilled and there is definate noise improvement. only low down in the revs tho, once your really gunning it its neglegable. The argument as to downsides is opaque as some would argue that it can allow warm air from the bay into the system however on the other hand there are people saying that it doesnt matter because of the turbo super heating it and the intercoolers cooling it down afterwards anyway.
my thoughts, if you dont have a turbo, worth making sure that you factor in some way of keeping the intake air cool. i.e. drill it if you wish but put a cold air feed to the general area. if you have a turbo i think its kinda worth doing the same as surely if the air enters system, goes through turbo and cooler, its better off going in cold than warm. maybe just not quite so significant. could suffer some heat soak when sat in traffic and pulling away. I drilled on the
side of the box towards the front/right (battery)