@Mark@Abbey m/s Think he's referring to fast idle emissions, which isn't usually affected by map, because it's always a light load 14.7:1 fuel target part.
@Nardo_Z I'd go for dual HFCs on each branch like I will be... I have a 3rd cat currently after the Y-pipe area which has a 5" mantle/body and 3" in/out, the largest you can get off the shelf. (It's a Jetex Euro 5) This still only just passes emissions for me, due to the ones installed by a custom exhaust fab shop likely being terrible quality - they wouldn't go less than 0.35% CO even when brand new. And the fact it's much further down the exhaust than it's designed for, so less hot/efficient -
Note that a single 3" in/out 5" brick/mantle is rated for around 350hp by HJS who make a lot of HFC for motorsport and supercar aftermarket systems. So my 3rd cat is also currently a potential bottleneck if I had an engine capable - https://www.hjs-motorsport.de/products/tuning/universal-catalysts.html
Looks like they now do a 3.5" / 6" model with 500PS peak and 3" / 5.5" with 450PS peak since I last looked.
Note - picked HJS info above because they have diagrams and power estimates readily available - they are very expensive and I can't find any evidence whether they are more effective at removing CO and HC than other decent makes. Personally I've ordered a couple of Magnaflow 3"/5" off eBay to add to my existing upper cats.
I'm aiming to get my CO% at or below 0.15% for peace of mind once I have quad cats like OEM. For reference I think a stock Z does around 0.02% CO with it's quad cats. (each side/unit has 2 bricks/mantles)