[84LX89GT]

What RPM were you launching with slicks? Most cars will bog a little with slicks and it also depends on what size slicks you decide to use. Also better gears with slicks would help a ton. You could probably get some awesome times by beating the hell out of your car, but i do understand why you ran faster with a little bit of wheel spin. Any time you spin the tires, that's wasted traction/power, though. If you can keep an engine in the power band and have traction, that's when you get your best times obviously.
I've gone to "import" nights at Pacific Raceways (closest track to me) and it's hilarious to see the imports with slicks. Redline launches and they still bog, lol.
I've never owned a V6 stang, let alone launched one at the track but i can imagine stock gears, stock torque with slicks would bog the engine. A guy at my work that has a '93 GT basically stock (ported intake and heads, exhaust, 3.73 gears...that's all) ran a 13.5, but was launching at 5500 rpm (this was a very cool day and he got perfect traction). He said he normally ran 13.8-13.9 range when he launched lower or during higher temperatures, but getting off the line gets the best times.