Note please that everything that follows is how I have done two others cars, and as my new 3 has not yet arrived, have yet to dissect it, but guessing most of the following is probably be what I would do my 3 if after removing the whiny/loud Dunlops, I feel that I need or want to do sound deadening. I am not suggesting you do all of the following, just giving your options, and first place I would start is lining the tire well and the flat area that lines around the outside of the tire well.
I have seen two posts that the panel that covers the spare tire wheel wheel is flimsy, with one mentioning he has covered it with a custom-cut round piece of thin (1/4" plywood or paneling) to stiffen it up. If so, the underside of that plywood or paneling would be one more area I would line with FatMat as my third priority.
Remove the vertical cargo area "side" trim coverings and do the wall behind them. In a sedan, one good place would be to line the under side of the "window shelf" -- the horizontal piece that connects the car's "C" pillars, just under the bottom of the rear window. I also saw someone post that he done around the inner wheel liners. (By removing the wheels, then the inner wheel liner, maybe easiest to do their "back sides" while the wheel liners are out of the car.)
Then of course, re-install all trim/covering pieces. One thing I have learned the hard way is not to ignore recesses/cavities inside the pieces I have removed. Most of these are so small that one can not go inside of them and line their sides, so I cover over their "openings." So if I pull off a trim panel and see some recess, say with a three inch opening, I will just cover over the whole panel with one continuous piece of FatMat (covering the openings outer ends if you will).
Note; FatMat will add weight to your car, so while I would consider doing one 20# roll (25 square feet), due to the Mazda's power level, I would not add a second one -- just prioritize the parts I mentioned. Make sure you do this at the minimum temp recommended (around 60 degrees if I remember correctly, and use a roller of some sort to press the sticky black side on pre-cleaned metal panel you are adhering it to (I pre-clean the received side with a 50/50 diluted IPA/rubbing alcohol you can get very cheaply at Walmart, Target, almost everywhere), but after applying the IPA, make sure you wait a minute or two for it to drying fully.
However, we all need to remember that no matter whether we also pull all the doors apart, remove seats, then the carpeting, and sound-deadening even those areas too, this will never be a Buick or a Lexis and be a quiet car. And I have never done any of these latter things (just mentioning lower priority options).