i have 2 8" subs in each of those cubbies. i think the vents are to release the pressure that arises from having the windows down and going at a high speed, and the rear hatch. i have gone 140+ with my windows down, with my subs, and no problem. nothing wrong with adding a little system, just dont go too overboard.
They are there for two reasons. One is to vent air to the outside if the air bags deploy. They account for a huge amount of interior volume displacement with an immediate pressure spike.
Reason 2 - Some of the very early GTSs had the rubber flaps over them stick shut. This caused rear windows to blow out at high speed when the pressure differential between the cockpit and rear bodywork surfaces was pronounced.
The vents in the rear quarter panels are for releiving the extreme pressure spike that arises when the shockwave arising from the exta volume of air being compresed inside the passenger compartment if your airbags deploy. Having those vents present may keep your eardrums from rupturing. On cars without cat's those vents are often what causes you to smell exhaust fumes inside your car due to the vaccuum that is created in that area.
Question.. Hard hitting bass makes a pressure spike also. If they are blocked and someone has a SERIOUS stereo in their viper, what would happen? A vented box with 2 12's or 2 15's moves in incredible amount of air..