I have been think about getting an intake for mine as well but wanted something CARB legal since I am in CA. You probably won't feel much as far as HP but I liked the Gen2 FIPK that I had on my 1st gen IS. The sound was awesome and the throttle response was a little better too.
chb402, do you have pics of your taillights? I have been trying to decide on the redout or tinting for mine.
The absolute best aftermarket intakes for the IS give you, maybe, 3-5 horsepower. On a good day. If you dyno it just right. Maybe. Some lose power under some conditions (from a dead stop really) due to sucking in hot air intake of cold like the stock setup.
In other words, save the $ unless you really love the sound.
Well I went with the Typhoon intake they make and its great, but I would scrap that K&N filter and go with an AEM dryflow. I have heard that the oiled versions leave a residue on the mass air flow sensor and could clog it, so thats why I went with the dryflow myself.....just a matter of personal preference.