Well, after 5+ years and 51,000 miles I have finally added a performance part to our Nissan Sentra SE-R SpecV. The first modification I made was replacing the stock exhaust manifold with an XS Power 4-1 Header. Everything else is stock with the exception of a short-shifter installed 2 years ago. Performance of mid to upper rpm range is noticeably increased versus stock with a definite improved pull to redline. The unfortunate side-effect of the new header is a definite rasp from the exhaust under hard acceleration. I guess a cat-back exhaust will be needed sometime in the near future 
Stock exhaust manifold versus XS-Power header
XS Power 4-1 Header for Sentra SpecV