Headers should I or shouldn't I....Need Advice Quick Please

[Mustang_GT_90]

Ok guys I got my car to a 13.90@98 pretty much stock, gears pulleys and a short belt, now the question is on exhaust. I've got a set of equal length shorty headers I've had them for a over a year and I'm thinking about throwing them on. My car is running the stock exhaust out the back right now, including 4 cats and factory mufflers.
If I say go with a 2 1/2" X-pipe and 2 1/2 inch flows and dump them at the axle along with those headers is that going to be too much and slow me down. My big concern is the headers, this motor is completely stock except for the pulleys and I've heard reports that without enough back pressure on them that it's going to slow me down.
I saw a dyno test on the web one time where they did a test with all the popular bolt ons including headers and an X pipe or H pipe on it and I can't find it anywhere. It seems like the headers were only really good for 3 hp and actually lost like 2 hp on the low end.
I'm getting ready to spray it 100 horse and I don't want my exhaust holding me back but at the same time I don't want to loose any power from all motor runs.
Thanks for any information, and if anybody knows where that dyno test I was talking about was online I'd appreciate it greatly if you could post a link.