Welcome to the forum...
I am also a V6 owner and hopefully I can help you out. I've got a lot of knowledge stored up so allow me to share.
First off I see you are running aftermarket 20" rims right? Are those the Kilis rims? yes those are heavier than even the stock 20" SS/RS rims and so that is hurting your time somewhat. I'd guess they are adding a tenth or two to your overall time for sure.
Are you running a tune? I'm guessing not, since tunes are just now starting to become available for the 2013. I hear EFI Live, IPF, and maybe even HP tuners either already have tunes available, or will soon. The LFX responds well to tunes....roughly 15 HP gain so I feel a tune is worth it.
So, you started off with a 15.5 and by the end you ran a 14.56. That is nearly a full second improvement, which in 1/4 mile racing is HUGE. Good job! Considering that the humidy was high, and the temps were a bit warm AND you have heavy rims, that 14.56 really isn't that bad. You have to realize, that where you track you car makes a huge difference in the times possible. In great weather at sea level, the SS V8 is capable of sub 13 second times. At other tracks with high elevation and/or hot temps, they can struggle to get much better than a high 13. Search google for a "DA Calculator" which can help tell you what you may have ran at sea level.
With your simple mods, your car should still be pushing around 340+ HP. Look at it this way. You car IS capable of a high 13/low 14 second time, but everything is going to have to be just right. The launch is going to have to be good, your shifting is going to have to be good, and the weather/temps will need to be good. There are guys that have run under 14 seconds in a V6 with JUST an intake, but they also ran in places were the DA was in the negative range....which makes this V6 very happy!
It sounds like you are getting the launch down. Launch at high enough rpm to get a little scratch off the launch and then get it on. TC off. (if you were an automatic...I'd tell you to leave it on)
If you can, lower the air pressure in your tires to around 30ish...some go to the high 20s psi. Don't be afraid to spin the tires and get some heat in them for the launch (again...if you want). Remove any extra weight (golf clubs! lol)
Best of luck!
5th gen IS SOLD! 6th gen built 10/14!!! Can't wait!!