99GT: mufflers or cat-back?

[91LX2Bfast]

My brother just pulled up in a 99GT auto w/ the anniversary package. Its is an sweet car and it only has 900 miles on it (damn I am jelous).
Anyway, I removed the air silencer for him and sent him over to the speed shop to grab a K&N, but I have a question about the exhaust.
He wants to have the rumble that my Stang had (2 chamber Flows and O/R X-pipe), so his first step is the catback. The factory catback seems pretty decent, so should we just weld in a set of new mufflers or is it wise to go w/ a new catback. I can't see that there is more then a couple HP in a mandral bent catback, so i think the money that would be spent on that could go to the H-pipe instead. What have you guys/girls done?
Ohh yeah, if he wants an H-pipe we are going to go w/ 2chamber Flows, but if he decides on a X-pipe, we were thinking of Dynomax Ultra flow SS welded. Does anyone have these and how do they sound? Thanks.