Mazdaspeed3 stock exhaust.  Have a muffler shop graft it in behind your cat:
I pulled those pix from earlier threads on exhausts when I was doing the same research that you are. I haven't taken any pix of mine, but it looks the same (except the welds aren't as pretty). There are a few threads outlining the procedure in the Performance/Powertrain section. I considered having the muffler shop weld a flange onto the cat, then being able to bolt the MS3 up and put the stock back on if I ever sold the car -- then I said forget it, too complex, needed a donor cat for the flange. <shrug>
My installation is virtually identical. At first, the exhaust is going to sound LOUD from inside the car -- but it's a GOOD loud, and it actually quieted down after a few weeks of daily driving.  Still sounds good, though -- guttural, deep, not raspy or "ricey."  (I use my car as a daily driver as well as my weekend autoxer.)
It seems to my butt dynanometer that I have more power and the car revs more freely, but I also get the sensation of having to wind it up more, revving higher before shifting in every gear.  But with the stock exhaust, I had trouble hearing the car while racing (yeah, I'm old) as I use the engine sound as my tachometer since I hardly glance at my dash dials while running.  No such problem with the MS3.
I bought mine from a member in the Buy/Sell forum; usually lists for $200 - $250.  Usually you will find it has been cut over the rear wheel arch for easy removal -- I just had my neighborhood muffler shop re-weld the cut section, then match it up behind the cat to stay stock-legal. It tapers out from about 2.25" behind the cat to a full 2.5" the remainder.
I considered others, such as the Corksport -- nice unit for the price, but you lose the cat and that's not legal.  Same for the Tsudo clones on eBay -- nice price, not legal.
(Clay, I remember you from when we ran against each other in CS during a Divisional in Gainesville, FL a few years ago, and from MiataForum.  You had the best time on Day Two, a 52.5 if memory serves, and I had a breakout second day, right behind you with a 52.7 even though I pushed a mistake through three elements on the course.  Enough to move me into fourth but not enough to overcome my lousy Saturday.  I had the yellow MR2 Spyder.)