Wow!!! High 15s and slower??!! Bone stock your cars should be in the low 15's. The tires, extra stereo equipment, and automatics are the biggest problems. Driver skill also plays a big role. The more you practice, the better you will be. It's tough being a newbie at the track but once you're past that part, you will do better (and then you can laugh at the newbies).
To compare, with only pulleys, cold air, x-pipe, and flowmasters, I was in the 14.7 range with a good launch on a five speed car.
I haven't been back since all of my mods though. I absolutely hate cleaning the rubber off of the quarter panels!