i was thinking about doing this. because ive noticed this car seems to retain a lot of the bass in the trunk. i once placed my box right in the back seat, and there was a HUGE difference from when the box was closed off in the trunk. ever since i had the urge to get the bass into the cabin better. i dont know if id make that hole in the armrest. i have an 05 and unfortunately its closed off from the trunk also. other models have the opening in the armrest to the trunk.
well personally, i dont try to make such big mods that i cant put back, so i wouldnt. what i just recently did (have yet to try because no system in again yet) is i removed the stock sub, hopefully that will help. removing the stock sub isnt permanent and it should definitely be of some bass improvement. you should try that before you cut that hole. i just built a custom box to fit in the trunk of my 05 gs so that itl sit on the gas tank and port through where the stock sub was. should be getting it aoll in soon and hopefully itl sound good, but i would recommend that you try taking the stock sub out first, then if you dont like itt or think you could get more bass into the cabin make that hole in the armrest.