And how important is dry hopping for added flavor?
Secondary is generally not necessary. However, for an IIPA, dry hopping is crucial. Based on research done by Stan Hieronymous, I now rack to secondary before dry hopping. If you leave the beer on the yeast, there is an interaction between the hops and the yeast that increases the levels of gerianol and give it (what is to me) an unpleasant floral quality. But that's one of the few times I use a secondary.
Secondary fermentation will help with the clarification of highly-hopped beers and help the suspended hops to drop out.
Dry hopping will add some aroma to the beer which is a desired trait for the style. Based on personal preferences, I'd recommend around 1.5-2 oz minimum for a 5 gallon batch. Let it sit at least 7 days.