well i don't have the srt anymore... i sold it a few months back. but definitely start out with exhaust, wga, tuning device i.e. safcII, motor mount inserts and an aem uego wideband. That will help you out a lot. What do you have done right now, if you aren't sure take some pictures of the engine bay and of the interior/exterior of the car so i can let you know what you have. Upload the pictures to a image hosting website like photobucket and use the img url code to paste in here.
ran a 12.11 at 111 on the stock turbo 1.68 60ft
DRIVE A 2011 Mustang GT 5.0 CS
425/397 JLT intake, Arh headers catless x pipe, gt 500 axle back exhaust, and bama tuner