Upgrading sub to get a live feel.

Watcher, hold on to your tater (more southern terminology) If you are going to upgrade to 4367 the subs you are using now will be totally lost.
That is one very efficient and very loud loudspeaker. Don't waste your money on stuff you won't be able to use in the future. To keep up with those speakers you will need something like two JL F212s at the least. Get your 4367s and use the subs you have now knowing you will go for the real deal when finances allow. Patience is a virtue. 

The 15 inch woofers in the 4367 cross pretty high up some where around 700 Hz I think. They are a pleated short throw design which will not go well below 40 Hz. Putting 2 high quality subs under them would clean up the lower midrange to a substantial degree and add that lower octave. You could also plug the ports which might make further improvements. I would rather two high quality subs than four cheap ones. Room control is a must for thewatcher with two subs as his room is very square. Make yourself a plan and stick to it. Messing around for the sake of messing around is a waste of money. 

Excuse me for the last post. I'm on some pretty heavy drugs right now and my mind is stuttering. The tie rods and struts can always be returned to stock. When was the last engine rebuild? You can always make it a looker again if you put the money into it but for a fun driver you just have to keep up with the basic stuff. 911s from 78 on are all galvanized so you really don't have to worry about the paint that much. It is just an aesthetic thing. The external rubber parts can get pretty moldy if the car was parked in the sun. They are easy to replace with modern rubber. Good that you have an indie you can trust. Not easy to find. Thing about 911s is you can make them go forever:) 

From a slightly different angle. Depending on the condition and value of the car, with 911s you do not want to deviate so far as to not be able to return the car to stock. Use drivers that fit in the factory locations. Do not cut into or drill any metal. Otherwise have fun. Most 911 guys will tell you that you already have the best sound system going, 6 cylinders screaming away behind the rear axle:)

Clio09, any 911 can be a valuable car as long as it has not been wreck, is not rusting and has not been modified. You want to use the factory mounting points for speakers and electronics that fit in the factory space. Outboard woofer enclosures are fine. No cutting or drilling metal! If the car is a renegade then the sky is the limit.