The speaker located between the seat is actually only ONE speaker (SUB) with the other opening being the port for that speaker enclosure. Changing the speakers and tweeters all around will make a difference, but without changing the head unit and adding an amp, the difference will be minor, and won't get you much more volume. There's a plethora of great quality speakers and subs available to fit the factory openings without cutting - but you should also consider Dynamat for your doors, to get more bass/snap/punch. I upgraded the entire package in my 08 Z06 by changing all drivers/subs etc...adding a DNX9140 Kenwood head unit and a PDX5 Alpine 5 channel amp with an Audio Control Epicenter (for extended deep bass). Sound is insane in that car now.
I'm going to do my 08 Viper 'vert this spring, but I don't expect anything close to the Z06 in terms of audio performance. Changing the head unit in the Viper to something good with NAV, will require a double din unit, which means cutting and painting the console/dash, but that will work well. You could go single din with a flip down screen, but you're not going to get the best head unit going that way - IMO.
