So, I've discovered that the speed shop near me will fabricate a custom cat-back system for me out of high-quality stainless steel for substantially less money than a pre-fab name-brand cat-back kit. All I have to do is provide a muffler.
I think that I've found something that'll fit my needs (deep, mean, but not too loud) and I was hoping to get some tips, advice, or a perspective that didn't occur to me.

So far, my plan is to eliminate the stock resonator and muffler (in the Mazda3, both are in series in the exhaust and the muffler is a round type, located quite a ways back from the outlet tip) and replace them with a Magnaflow XL 6" Round 3 Chamber Universal Muffler that's 27" in length. I'll be using 2.5 inch pipe.

So, what can I expect from eliminating a resonator and using a large muffler like this?

 

Well, I have no problem with using that muffler, as I have a Magnaflow XL on my Civic and I like it a lot, but....I would throw a resonator in there if I were you. I don't really know how it will sound without one, but when I had my custom exhaust done I didn't have a resonator for about a week and it sounded so bad and raspy I didn't even want to drive. So I went back asap and had one put in there. Sounded much better after that. But like I said I can't predict how it will sound so thats just a risk you will have to take. Good luck though.

 

put a resonator on it... that's it

 

Well, good news:
One of the distributers linked to from the Magnaflow website is offering the component that I was looking for at nearly half-price which allowed me to purchase my desired muffler AND a 4"x14" round resonator all under budget!

I should be getting the system assembled next week and I'll post more about it then, maybe even with a sound clip.