Cobra intake was worth 5 mph for me, but not much time. My '97 ran a 15 flat, at 100 mph, with an intake and exhaust, and a cone air filter and a tune. Over the winter I swapped to a PI converter, long tube headers, and 4.10's, had the tune redone, and it went 13.9's at 100 mph. I could never do better than that. It was all dependant on my 60' time - a 1.9 got me a 13.9, a 2.0 got me a 14.0, and a 2.1 got me a 14.1 - every time.
The last winter I had it, I had the exhaust totally redone with all mandrel bent tubing. Never got to run it after that, had to sell it after I got my Navigator.