I just got a 06 Titanium Gray Mazda 3 S 5-speed last Friday night. It's a base model S with nothing else on it. I love it so far. It is fairly different than what I am accustomed to. I owned 2 Mustangs before this.
And actually both of the Mustangs took 87 Octane as well (before being modded at least). But there is a fairly big difference in gas mileage.
Not any performance mods are in the near future. I got this car for a good daily driver. I do definitely plan on tint very soon, and maybe a sub/amp to add a little bass.
I think you will find that the stock speaker set up kinda sucks. The speakers are ok for stock, but the door panels rattle way too much. Dynamat may be something to consider as well.