With cats would muffle the sound, hence why they say O/R H or X is not for street use=louder. Less restriction will equal louder note, however, there are several parameters that will change that, manufacturer, material used, etc.
Now what is your goal? Loud, raspy, or more of a muscle car sound? What series FM's do you have?
I won't be that guy to tell you what brand to buy. Your best bet is you tube and search different combinations.
I personally want a Mac P/C, but just haven't had the dinero or the time to put towards it.