anything decent is prolly going to be a big improvement over the stock drivers, as long as the new ones don't require a lot of power.
the stockers are obvious pretty low cost, although there's something to be said for some lightwieght junk and certain frequencies in the midranges. meaning they can produce a lot of sound for what they are, although not necessarily with outstanding clarity or accuracy. still, in a car as loud as my hatch, i'm not expecting perfection, just good tunes and loud if i want it.
it's nice to be able to play the stereo loud when tailgating, so you can play catch from far away and still hear it. but i don't try to impress people when driving.
i had awesome all alpine systems, with custom enclosures i built myself, and Fujitsu ten drivers, back in the early 80's before it became a professional thing. having all that personal work stolen more than once tends to take the edge off the need to be impressively loud. plus it's not good for your hearin.
i can't decide between a stealthy improvement or a removable sub/amp combo, so I don't lose any space.
as big as the hatch is, i must admit it's a struggle to fit some of my friends golf clubs. meaning the limit is 2-3 passengers.
I haven't heard the KAPPA's but based soley on freq response charts I would go with the CDT 61a's without a second thought. The CDT drivers are very flat down to 80hz while the Kappa's have a steep dive that starts at 100hz. The CDT's are also cheaper. Here's the package I bought from woofersetc.com:
A forum community dedicated to Mazda 3 and all Mazda vehicle owners and enthusiasts. Come join the discussion about MazdaSpeed performance, modifications, classifieds, troubleshooting, maintenance, and more!