I'm not splitting hairs, but it is 264.5 depending on too many factors and how you want to view it. I was pulling the 258 from a different run. Still pretty good with 2/3 of the tuning down. On a side note, Car and Driver had an article about dyno runs and the errors that exist in getting actual hard data, every chassis run is basically a crap shoot.
Check it out, interesting stuff. The good news is that your car may be actually losing as much as 30% through the chassis. 15%, even for a 911 is on the conservative side.
http://www.caranddriver.com/article.asp?section_id=4&article_id=8020&page_number=1