high flow cat ????'s

so my 1990 5.0 GT stang is too damn loud and has no cats on it which is illegal in California and wondering two things:

1. will putting high flow cats quiet my car down a noticable amount? right now i have the stock catback on and would like to put my mandrel 2.5 flowmaster setup back on but am scared of getting pulled over, stock mufflers= slowwww and no cats=huge fine
how much will it quiet my car down?

2. what are the cheapest cats i can find that are high flow 2.5 inch, i found these ones that i guess could bolt between my X pipe and my catback and avoid paying someone to cut and weld them into my x pipe? will i notice any loss of horepower when i put these on? my car still has stock heads and stock intake (not for long........ )

http://www.machv.com/busrachigcat.html

maybe someone can recomment what the cheapest muffler around is that flows equal to my flowmaster 2 chambers??