These EcoBoost engines don’t really gain much from exhausts or CAIs — they’re already designed well from the stock form. Those upgrades are mainly for sound/looks. If you want real performance gains, tuning is the easiest way to add 40–100 hp. After that, you start getting into the bigger stuff. Prob the two best "firsts" that make a usable difference on these are tunes and a rear sway bar.
I'd have to update your comment to "tuning and a intercooler upgrade". It is a waste of money to throw a tune on a stock ST without first upgrading the intercooler if you ask me. The stock IC is ridiculously under sized and will heat soak the first 100ft of a WOT run causing the engine to detune itself to compensate for increased intake temps really hampering any tune.
You can do an IC, tune, rear swaybar and you have 90% of the performance increase and best bang for buck handling increase you are going to get without touching the fuel system and turbos or other fancy suspension parts. Edit I have to add subframe lockouts, they are AMAZING for the ST!
For OP if you just want sound, go cat-back and call it a day. Exhaust other than also doing downpipes with other supporting mods won't help your performance. If you are really chasing actual performance then what I typed above stands in my book anyway.