Doors are a great place to start because the results are immediately apparent at ear level - both in the total road noise passing through the door and the satisfying "thud" when close the door. You will definitely appreciate it the next time you pull beside a noisy diesel truck. You'll almost hear the diesel racket through your floor since the door is so "dead".
I'm not completely convinced about A-pillars. For the effort and time, just go directly to the floor. My friend is planning on tearing out the carpet and immediately bolting the driver's seat back in. That way he can still use the car while the project drags on.
We determined that the mat was only useful on metal that "rings". If it simply thumps when you tap it, don't waste your mat. You'd be better off putting down the ensolite or padding. For the roof it might make sense to use mat to reduce it's ringing sound - but getting behind that headliner on a sunroof car won't be a lot of fun.