I have read the 4.6 tech that they worked just fine for those who have purchased them. They said they made their cars sound good and provided some hp gains as well. I personally have not heard/seen or know anyone running them.
i have the mac pro-dump setup it is 2 1/2 mac flowpaths dumped with flow tubes, everythign bolted up real nice. every body i talk to loves it. INCLUDING ME thats #1 go for the sound you like