The A pillar area to be concerned with is the lower portion behind the kick panels. Pop them off and you will find under a bit of padding(or not) access holes into the pillars that can allow a fair amount of noise to enter from that location in the vehicle, near the front tires, etc.
All that needs to be done is use a patch of mat over the holes then a bit of foam over that. Not a big job and does not make a night and day noise difference but certainly worth doing it, especially considering how little material and time it takes
I agree to a point, the mat is Most Usefull on the resonate areas and when doing a minimal weight gain install they are the areas to mat, leave the other areas alone, then foam over all areas.
But, if you are going all out and want to gain every edge you can, just mat the whole floor and then go back and use a second layer over the resonate places, a good old thumb with your fist will let you know where it is needed. Then foam over all areas as above.
There is alot of high energy resonance that travels through the frame rails, pillars, etc, in all cars, especially those that come from the factory with very stiff suspension bushings.
Another level of deadening/chassis stiffening is obtainable with a great deal of work and a bit of cost, foam filling all voids throughout the car. We have completed our project car and it was quite an undertaking, it is a labor intensive, messy, just not fun job to say the least, lol! We just supplied a shop in the Portland area with the materials to fill to cars, one a totally built, stitch welded and fully caged WRX race car, the other a customers car. We are not in the business of selling such products but have access to a foam that is somewhat resonable and seems quite effective, sure made our car stiff!
I will let you guys know how this goes when I get feedback from them on their experiences, after checking out our car they really wanted to give this a go around.
Our car is going in next week sometime for a back half welded in chrome molly cage designed to be expandable to a full cage just in case we decide to make this a full race car sometime in the future. (Unfortunately not a Subaru