I have the Solo Street Race with no mufflers. It's not loud. The turbo acts as a muffler for the exhaust. I will say that if I go to a HFC, that I might change my mind on that.
Small gains only happen after the large tune gain. Nothing will get you more then several hp here and there after your tuned. GM did a really good job letting the car breathe. The factory exhaust and intake really don't need to be replaced until you add a larger turbo. And you'll never get more then around 350hp from the k04 turbo. It turns into a heat pump after 24psi of pressure. ZZP tuned mine to 24psi of boost, but my fan comes on to cool things down a lot sooner then everyone else's. My fan starts at 182 degrees in goes up 20% for every 2 degree increments. So I'm full fan at 190 degrees, where factory and most tunes are full fan at 220 degrees.
So here's some info to chew on. When I had my car dyno tuned at ZZP, everyone there had either a Ford Eccoboost car or a GM turbo car. Most were the Camaro and Cadillac with the 2.0L. The tuner said for $1800 on the LTG motor (3rd gen LNF) that he could make 400hp easily. That $1800 would include, HFC, IC, IC piping and CAI. And then of course the dyno tune cost. He said the eccoboost by Ford was even easier. He said no mods other then a CAI and they could push almost 600hp. I must sadly admit that I accidentally got smoked one day by a Taurus SHO in my Scat Pack Charger.... And as SHE passed me, in the back window was tuned by ZZP.....
DDM Works, Performance Auto Werks and ZZPerformance also offer nice kits. I would check them out too. Make sure you do your research on all of them before you make your final decision. Quality, customer service, "bang for the buck" and helpfulness should all play into the decision.
Here's a quick vid of my car being dyno'd. Go to 0:34 to see how much air is actually pushed out the back...watch the banner flap in the breeze from the exhaust.