The way I look at it, I want maximum bang for the buck. If you can replace the mufflers with something better, that's probably going to give you a lot of bang for the buck. I'll just say that unless you have some serious mods, I doubt that enlarging the pipes or going with a high-flow cat is going to do much good; I think the one dyno I've seen for the cat mode only showed something like a 2 or 3 hp improvement? That's probably >$100 per HP, and probably on a car that has a lot of other mods. Much cheaper to replace the mufflers and the header. (Or even Hondata, going with bang-for-the-buck?)
Also, I'd leave the resonators in. I'd question whether they would be worth removing (they should just be straight-through designs), and they'll reduce the droning while crusing, as far as I know.
I went through all of this on my previous car. Some of the aftermarket kits have small resonators, and the noise was horrible on one that I rode in. I bought the "quietest" that I could find (which was still a bit louder than stock, but mostly when you got on it), but at crusing speeds, it was pretty decent. It had a good-sized resonator.
So, in short, I think if I wanted to change the exhaust for my TSX, I'd probably do the header, and maybe just replace the mufflers, and that's probably it. Although, right now, I don't have the bug to mod much, and this is all kind of my guesswork, so take that for what it's worth.
