the collector size is different, the JDM systems use a 2.5" collector so yes you will need a different CAT
BUT
the jdm Cats are much shorter, this is not due to the manifolds length but the exhaust B-pipe being longer
so ideally you would have to run
-EK9 spec manifold
-JDM CAT (or jdm spec decat)
-JDM spec cat back exhaust, or depending wheat system you are running then just a JDM spec b-pipe (spoon b-pipe is ideal for this as it will mate up to any oem fitament back box)
cannot comment on how low it will hang i'm afraid, although 4-1's generally hang lower than a 4-2-1 which aint so bad