I always have to jump into a thread where the car didn't run what they thought it should.
Yup, that's the real world. Running quick at the track takes a LOT of practice and a well sorted combination.
My first comment: Most of the mods mentioned in the linked are appearance or have little to no effect on power. You may have gained a few horses from the o/r x pipe and upper plenum, otherwise you dang near stock power wise.
My second comment: lowering springs will do a great job of killing your launch, no matter what tires you are running. They screw up the rear
suspension geometry, lower your center of gravity and reduce weight transfer on launch. You can blame the track prep, but how many places do they really prep the track well for test n tunes? My GT would do 2.1 60' times with a poorly prepped track, on 3.27 gears, with ALL SEASON tires (245 width btw). Bottom line, lowering the car will hurt the 60' times.
Last comment: What rpm are you shifting at? You said that you are shifting to 4th just before the 1/4 ends, but yet you have 3.73 or 4.10 gears. That doesn't make sense at all unless you shifting WAY to high. On the 1-2, you should rev it out to 6200 if it actually hooked up. On the 2-3 and 3-4, most PI owners find ~5700rpm works well. Beyond that, you are hurting yourself. In your other post, you said the car has a 6500-6700rpm rev limiter, but that is much higher than you'd really want to shift. In fact, I wouldn't even spin it that high on the stock rods. I bet you'll pick up mph when you start shifting lower. Also, power shifting helps, but compared to a good speed shift, it will only make about a tenth of difference.
As a reference, my NPI ran 14.2 on street tires, and 14.0 on a nitto DR. I couldn't get it to run faster than a 2.0 60' time on 3.27 gears, no matter how much I abused the clutch.