Only if you are in a state which does not enforce "CARB" style rules. The Katana2 is essentially the MP62, which is the supercharger used for the Exige. In all fairness, many emission test facilities might not be able to tell the difference, but if you have bad luck, you might be the one they figure it out on.
Agreed with what addertooth says above. I live far enough out that I don't have to do emissions for the Seattle area. I have a NA Elise and absolutely love the power bump after the install. The katana puts down to the wheels what the Elise SC does at the crank.
If I had a stock SC, I don't think I would go for the Katana kit. To me the HP to dollar ratio doesn't work too well. Though I'm sure you would enjoy the increase in power throughout. I would go for one of BOE's kit with a TVS. It's not that much more money for the 300 kit. Not to mention the modularity of it and tons of growth room from that base package.
I've had my kit for just over a year now, I'm already looking at BOE but it's hard to justify that leap after already having the Katana.....if that tells you anything. The kid inside says screw Spock and your logic.