If the wheel and tire you're upgrading to weighs more than what the OE one does than there's no reduction in the unsprung weight.
My Advanti 19x9.5 wheel and 275 tire combination weighed 4 lbs less than the OE 18x8 and 235 tire. The only way to find out is weigh them.
You could try these clamps from Ford, they're the same ones used from the factory to attach the catback.
https://secure.revolutionparts.com/levittown-ford/ford/mustang/br3z5a231b/2015-year/gt-trim/5-0l-v8-gas-engine/exhaust-system-cat/exhaust-components-scat/?part_name=clamp&position=left
I have for sale my Corsa Sport Catback with polished tips. Part # 14332. It was used for less than 300 miles. It's complete and perfect condition. The slip connections are slightly crimped from tightening down on the clamps. SOLD
There aren't any black OE rear decklid badges available . So I went with the blank Ford Racing decklid panel.
http://www.levittownfordsupercenter.com/mobile/ford-racing-mustang-tail-lamp-applique-p-3462.html
I have those and the grill pony on my car, I also bought them from Levittown.
http://www.levittownfordsupercenter.com/mobile/ford-mustang-emblem-pony-black-p-4250.html
I bought mine in August and it was one of 3 or 4 2015 GT's on the lot. It had a huge discount on it plus cash back from Ford and 0% financing. A white automatic wasn't my first choice but after I test drove it I had no doubts. Plus it's nicer driving in traffic.