IMO if you're comparing your Z to a 2011+ Mustang you bought the wrong car. You're going to lose the HP battle every time unless you're FI and he isn't.
Sure, a full bolt-on Z *might* hold its own in the 1/4 against a stock 5.0 with a sh*tty driver. So what? What happens when the 5.0 gets 300 series tires, control arms and a tune? You're comparing a V8 muscle car and a V6 sports car, two totally different types of vehicle.
If all you want is speed get a Mustang. Period. End of discussion. Want it to handle? Buy some coil-overs and brace it up; it'll wee-wee on your Z around a track. Need even more power? Add a Vortech and flip off the GTR drivers as you pass them.
Know why I don't have a Mustang anymore? Because they're all over the damn place and there's a lot more to owning a car than how fast it goes. I'm sure there are EVOs and STIs that can hand a Z its @ss as well, but at the end of the day they're still just economy cars with AWD and turbo.
If you can't appreciate your Z for what it is, sell it to someone who can.