Not to be a tree hugger here, but if you get a good high flow cat, there is little to no loss in HP, you are emissions compliant, you're not polluting, and it makes Baby Jesus giggle! LMAO, but seriously, any HP the stock turbo/ecu will let you make is way less than a good cat can handle.
As a testament to this, I'm not in an emissions state either but and when my Pope DP comes in, I'll be having a high flow cat installed when I have my mid-pipe built for this exact reason. Plus if like me, there is talk that the state might start to require emissions, It'll cost me more to make it compliant than if I'd had it done when it was built.
The flip side to this is I tried to install a cat (3") on my 240sx, and at about 300hp, it started costing me a noticeable amount of power. At 400hp, it melted.