Yes but you have to be careful not to melt the stock pistons or break a ring or ring land. Aside from that, the nitrous oxide can certainly boost the torque, and to some extent, the horsepower, but the big horse numbers come with improved rpm capability. If you can only take your I-6 up to 3500 rpm or so, that's your limitation.
I like how the article attacks the I-6's problems head on, by improving and upgrading the strength of the rotating assembly, and improving flow through the head. That's huge. That with a good cam to optimize your newfound rpm range, and you can get a bit better than 1 hp/ci. But I don't think you'll get 400 horsepower without squeeze or some kind of forced induction.
