be carefull with proper sizing of cat !
there should be BHP ceiling recomendation, from core manufaturer ... eg. 100mm diameter, 200CPSI core sport cat is suitable for 150-170bhp max, othervise serious increase of backpressure will occur, (as I had on my 2l 16v NA with 100mm dia/200cpsi universal sports cat and was able to rev up to 3500rpm/WOT without problems..., but turbo engines are not so sensitive to that.. ther will be only some lost of power...)
this backpressure could cause engine running slightly leaner...
so for 400bhp u need at least twice area of core, or smaller density... so at least 130mm+ core/100cpsi should be good for small amount of backpressure