Intro and Headers Question

Hello everyone. I have been a member over at chargerforums for a little while now and I decided to come over here and post.

I am a US DOD contractor living and working in Seoul, South Korea. I was in the Army for five years (finished in 2003) and I have been a contractor for the past three years. I am 26 years old and recently married. I will post photos sometime later. Anyway.....

I am thinking of ordering headers within the next month or so. My questions are:
1. Do long tube headers have any negative effect on ground clearance?
2. With headers and hi-flow cats (I have magnaflow cat back and K&N CAI) what HP gains can I expect? (I am thinking 15hp).
3. I understand about the problem with the CEL which is ok with me as long as it doesn't have a negative impact on performance, so does it have an impact on performance?
4. Lastly, any recommendations? I was thinking of going with Kooks....

Thanks for all the help.