id say that apart from making little difference at the wheels, the only reason i would be changing to to a metal CAT is they are less likely to "meltdown". if a heap of raw fuel is punched down the exhaust and ignites and melts the monolith, then ofcourse going from a pile of melted (blocked) mess to a highflow you will notice a difference.
also metal CATs are arent known for cracking up and coming apart like ceramic CATs, including impact damage for those who like their cars low and their CATs eating speedhumps and driveways