The difference is with an un catted mid pipe your downstream 02 sensors will trip a DTC code for catalytic converters inefficiency. The downstream 02 sensors can be turned off with a tune, other wise they will likely turn the cel on. Not to mention if your state does a visual inspection it wont pass. Advantage is there is no restriction in the exhaust from cats, cheaper, and they will give you a louder tone.
High flow cats are just what they suggest, they flow a lot better than stock cats do, advantage is you keep your catalytic efficiency , will be legal to run with slightly more resistance in flow . They are a bit more expencive.
I just purchased an off road xpipe through summit racing for my 2000 gt with the down pipes for the stock manifolds , cost was $168.00 The ones with high flow cats were significantly more expensive, some where in the neighborhood of $320-$600
Last edited by dawson1112; 06-11-2014 at 08:25 PM.