New member, long time reader. I registered just to answer your post because I had the same issue.
I have a base 13GT Cab (vert) automatic that I installed the premium cluster with track aps and 18x10 rear wheels. The best I could get for the 0-60 was 5.0. No matter how I tried launching, I could not get the tires to bite. Last weekend I installed BMR LCA with the relocation bracket, and man what a difference. I still can't go WOT from the get go, but I can get 70-80% even in 40F weather. With that change I was able to consistently get 4.7s on launches this weekend. The transmission is set in "S" and I am not doing the manual shifts as they are not quick enough to prevent redlining. I am hoping that when the weather warms up I can go WOT and get the 4.5s. My thinking is that a tune to set the 1st and 2nd gear shifts to 6,900 rpms can get me another .1 and then 3.73 gears can get me to the low 4s.
Just curious. Are you lowered? If so how much? What holes did you use in the bracket? For stock height I went 1 down from stock. For 1.5" drop I went 1 more hole down. I have not tried the bottom hole.
But as a warning, the lower the hole you use, the more torque/twist is applied to the axle tubes and thus the more force to resist at the connection to the pumpkin. I would suggest getting the axle tubes welded to the pumpkin. Special weld material and methods are required due to dissimilar metals. If the connections break loose then you will likely damage your car in the wheel wells etc. and expensive repairs.
And in my opinion .373's with the stock automatic is too aggresive. There are threads on this that you can search. You are better off with an aftermarket torque converter.
2013 GT, Track Pack
, Recaros, Kooks Axlebacks, Airaid CAI, BMR Rear Adj. LCAs, BMR Relocation Brackets
, JLT Oil Separator(s), Barton 2 Post Shifter Bracket
, Steeda Motor Mounts
, Barton Shifter
, JHR Clutch line, BMR adj. UCA and Mount, Kooks 1 3/4 headers/catted H, DSS Al Shaft
, BMR Loop
, Vogtland Sport Springs front/BMR rear, Strange Adj. Struts/Shocks, Extended ball joints, Steeda Bumpsteer Kit, Whiteline Tranny Mount Bushing, Borla Overaxles, BMR Watts, Vorshlag 3" Ducts