Throwing out some other ideas. Another popular product used is the 3M thinsulate material.
From the website: Delivers more acoustic absorption per unit weight vs. competitive products. Helps customers meet mass reduction goals. Non-woven sound absorbing fibers create a quieter vehicle environment. Hydrophobic fibers resist moisture, mold and mildew.
There are also similar generic types of thinsulate products on ebay with an adhesive backing. Parts Express has a cool 3-in-1 (butyl, aluminum and cc foam) product. I've also used their Sonic Barrier vinyl sticky sheets since they are easy to apply and look almost factory. Thick carpet floormats with rubberized backing can also be a nice incremental upgrade, I think that was verified in the movie Die Hard. Speaking of carpet, I wish we could buy some nice thick aftermarket carpet. I dislike the stock molded plastic stuff with the "saltbae" fibers sprinkled on top.
To keep the car serviceable I would go with strategic placement of butyl sheets (esp where the OEM has already identified) and a good amount of thinsulate behind any of the plastic panels to start. MLV is proven but it's a significant step in my opinion so I'd probably do that last or get different tires, then go MLV.

Tip: Adding a couple of sticky squares to the backside of super thin plastic panels can give it a more substantial feeling. Could be something like the door panel or shifter surround. Eat that bean counters!