if they are glowing they are definitely bad, you can go for an off road h-pipe(cheapest route if you dont have to pass a smog test) or you can pony up the extra cash and buy a new h pipe with high flow cats, or pick up a junkyard unit good place for used parts is
http://www.mpssalvage.com they are the largest late model mustang dismantler i have used them before with good luck
also might want to look into replacing the O2 sensors(2 of them) alot of times failed cats are from bad o2s that made the car run rich they run about 40$ a piece autozone carries bosch parts these are fords oem supplier for the O2's so id pick them up from there best price i have seen.