I think the commercially available HFC are a big limiting factor to turbo cars. Those of us who choose to use catalytic converters to reduce smell, sound or emmissions need something better. When I installed HFC on my JWT TT I felt a huge loss in power and response. Before this I had test pipes but they would stink up my clothes. I use my car for work and like to roll with the windows down so test pipes are out.

I think a 3" to 3.5" inlet to match the downpipe and a much freer flowing catalytic substrate would reduce backpressure tremendously.
Metal'cat makes a 3.5" universal cat with as low as 100 cell per inch substrate.
Most HFC are 2.25" to 2.5" and use 300 to 400 cell per inch substrate.

So a HFC could be made with an inch more volume and material with a third of the resistance.

Heres a link to the FI thread I started.

https://my350z.com/forum/forced-induction/275005-3-5-custom-fi-catalytic-converters.html