Paul
First, fitting hiflows should NOT put the CEL light on, not unless a lambda probe has been damaged removing it, or one has just decided it's had enough, they are really tight to get out of the cats, and can be damaged if not extremely carefull when removing.
Secondly, the Kinetix cats are a lot lighter than the Nissan ones, and as such do not require the brace bar, just keep it in case you sell them in the future, and wish to revert to the originals.
And yes they do sound awesome, especially with an aftermarket zorst enjoy Paul. ps, try an ECU reset
Ian SATS