DO IT. The car will be so much more fun to drive everywhere except the interstate freeway at speeds over 75-80mph or so. But how much time do you spend there? This is a Roadster, a sports car, a fun car. It's not a grocery getter, a commuter, or a hyper miler.
DO IT. You won't regret it.
BTW, you cannot install Posit-Traction if you're using a Ford 8.8 differential. Posi-Traction is what GM calls their limited slip differential. Ford calls theirs, "Traction-Lok".