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Other than that, on a C4 you'll need headwork, cam, computer tune, and a good exhaust set up. Depending on the combo of parts you should see an extra 100+ horses and in some applications more.
as long as you have a good tune for the nitrous and don't spray it all the time, the engine life shouldn't be affected much.
For 100 hp, you can do the nitrous, which is the easiest and cheapest. You can probably gain 100hp with a supercharger, bigger injectors, and a good tune, not easy, not cheap. Or you could do a head, cam, intake combo for 100hp or more, but you'll need injectors and a good tune, again, not easy, not cheap.
But it is the cheapest way to get power, that or other means of forced induction. Head/intake/cam work is expensive for such a lesser payoff in the end.