You're right. Not all 16-year-olds are immature drivers. And not all adults are mature drivers. Only a case-by-case assessment can truly indicate the maturity level of any one individual.
That being said, the overwhelming statistics show that driving experience has a direct correlation with how well a person reacts to any given driving situation, regardless of "maturity." I imagine as the power of a vehicle increases, the sharper that correlation becomes.
Most minors have not been driving vehicles since they were 4 years old. So, you're right that we don't know how capable the OP is behind the wheel of a 400hp vehicle. But at the same time, neither do you. You're whole-heartedly sticking your neck out for someone you know just as little about as the rest of us.
I'm not going to say whether a parent is irresponsible or not because they gave their child a powerful vehicle. A parent should know the responsibility level of their child before they give them anything. I sincerely hope that the OP's parents adhered to this simple truth before handing him the keys to an SS and said, "have fun." I have no evidence to indicate otherwise.
So, rather than argue about whether it is responsible, intelligent, smart, stupid, etc. to give a 16-year-old an SS, I will tell the OP this:
If you're going to get an every day beater, take into consideration the climates you'll be dealing with. I don't know how much ice and snow Pilot Point, TX gets on average per year, but if it's significant enough, I would suggest a vehicle with 4WD that has a fairly even weight distribution front to back. Trucks typically are a little more squirrely on ice and snow simply because they don't have that much weight over the rear axle. An SUV with 4WD would be a great choice, but your budget may limit your options. I love my '95 S-10 Blazer for inclement weather conditions. Plus, if anything should happen (like getting in a fender bender) I'm not really THAT broken up over it.
And last but not least, you better show your parents how appreciative you are for giving you not one, but TWO vehicles. Not many 16-year-olds are that fortunate. And, typically, the ones that are usually don't recognize how great they've got it.