Aftermarket cats = high flow (HF) whether they are the ones that bolt up to the stock exhaust manifold (Solo or JBA) or cats that bolt onto long tube (LT) headers.
Simply stated, the benefits are in the name; they flow the exhaust gases better than the stock cats. They also keep you emissions legal except for more restrictive states like California.
Kooks 1-7/8 LTs; Calico coated w/HFCs, Solo Mach XL catback w/J-pipes, VMax TB, C.A.I. Intake, JRE Tune, RX Catch-Can