I thought I'd add another post to this thread. There are many posts like
@JeffSpicoli above and between that post and today, I've seen scores of similar posts. There seems to be many people who think the first thing one "MUST" do to achieve performance is to swap the factory exhaust and intake. For the ST, I completely disagree.
But where does this idea come from? Vendors trying to sell exhausts? People just wanting more vroom? Or something else?
Let me put this forward- it's because the ST is not your Dad's (or your's) 20-30 year old turbo vehicle. The conventional wisdom about older vehicles (perpetuated by vendors, and those who bought the products) is that there are huge performance gains with an exhaust and intake.
What you might not realize is the technology was nowhere near where it is today. Any idiot could muck about with an aquarium valve to screw with boost pressure. You could also play games with mechanical adjustment of the wastegates; and buy manual or even electronic wastegate controllers. Which would work...until you lost that rod bearing or piston.
No manufacturer wants pissed off customers, nor do they wish to replace engines under warranty. So, what might be a solution in the days before the sort of technology we now have in a turbo vehicle?
Simple, use the intake and exhaust as additional boost control mechanisms. A restrictive exhaust and/or intake provides a little bit of insurance to the engine when someone ignorant plays boost games.
This then led to the idea that you "had" to change the factory intake and exhaust to make gains- and this was correct thinking at the time. Of course that high-flow intake and exhaust led to thousands of destroyed engines since that very restriction was part of the manufacturers' boost control strategy. And once you overrode that capacity, unless you had a good tuner, you were also overriding the injector capacity, the timing correction capacity, the MAF sensors, etc.
The ST is not in that category. The intake and exhaust are NOT a limited factor at 25-35% over the factory HP (mass airflow) rating.
If you go above that 25-35%, you might need to think about playing with the intake or exhaust. But to achieve that, you'd have to replace the stock turbos. Even then, mathematically, the stock exhaust system should easily be good for 650+ HP (that doesn't include the downpipes which I would replace along with the turbos).
Now as posted earlier, if you are only interested in some acoustical gain- buy whatever you want, it's your money. But don't post about butt-dyno HP gains (and God forbid MPG gains) from intakes and exhausts and expect to be taken seriously.
agree on diameter not being a restriction, there are gains to be had with Down Pipes. I’ve seen at least 5 independent Dyno runs showing gains of several different Down Pipes. The range was 20-27whp. I’m sure this is dependent on power level, as there is likely no gains at stock power levels. 
