about the deck lid spoiler. I got myself a replica one that was unpainted for 80$ and just put vinyl on it and used 3M double sided tape so no drilling required. cheaper than buying the OEM.
i also installed the under body panels and believe they do help. you do need to drill holes for 2 of them but i didn't and they are holding just fine
heres a link to my car and what i have done for some ideas. its not much but something
http://ecomodder.com/forum/showthrea...ing-17911.html
could you post a picture of the rear fender liners?