Okay "high flow cats" is a subjective term. First of all all cats here in the US that are aftermarket are all honeycomb type. These so called "high flow cats made here are no better flow per area. They usually just increase the diameter of the cat to get a bit more flow. Difference in power is probably unmeasurable. It is the same technology they've been using sicne the 70's.
Now Emitec out of Germany came up with a new design that outflows honeycomb by a long shot. It is called a metal cat. Metal substrate. Very cool stuff. I experimented with it many years ago but found cost too high to market successfully. You can actually see right through it and has much high thermal resistance too (important one some turbo cars). Anyway, They are what all the Japanese companies use for high flow cats which is also why they are so expensive (usually in the $500 range for one cat with flanges). Now, it still can not outflow a test pipe but it is nice and keeps things very clean. It is used by some manufacturers for certain cars. Mercedes and BMW use it as does Porsche and Ferrari. Even Honda uses it on the S2000 but not sure if the Z33 has it or not. My guess is no. So anyway, if you look at the price tags of some of these Japanese cats and find them pretty darn high priced, that is why. Hope this helps. That being said I'd go test pipe if possible.