You need to take your car to the drag strip to get the actual #'s. If your car is running better than 15.20's, then I'd say your "tune" is working.
The best thing you can do to improve 1/4 mile performance is install a NeedsWings CAI. It will cut nearly a 1/2 second off your times.
Or simply install a "smaller" set of rear tires, or do both.
Motor Trend Magazine did a comparison road test of an 04 XF and a 03 SLK in their Dec. 03 issue. Even though the SLK weighed 70lbs more, it still ran .42 sec. faster thanks to its lighter 225/50/16" tires that are more narrow than the standard XF rear tires.
I would also recommend that you disconnect your "Traction Control" at the Drag Strip.
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Just turning it off, doesn't completely shut it down. One thing you don't need at the drags, is your computer braking, or cutting power just to improve traction.
Most any SRT owner will tell ya, the NA cars aren't drag racers, it's not their forte, and they would be correct. As I mentioned in a previous post, our cars would probably run in the 13's with nothing more than having a 4.10 gear, a 3.27 is just too high for serious drag racing. (But then we wouldn't have 150 mph capability, as if that's important here in America).
That being said, the NA's 1/4 mile performance (on average) matches most all the stock American Muscle Cars of the late 60's with a similar gear ratio, and around 400 cu. in. engines. So even though the best "Pure Stock" times of the SRT vs the NA are 1.4 seconds apart (according to automotive magazine road test) It's not terrible, all things considered.
It's just that the SRT's have so much more potential, it simply doesn't make economic sense to throw ton's of $$$ into an NA car for Drag racing, unless you really, really can afford it.
But that's a decision each individual owner has to make.