Not true at all. The reason the power slowly fades as you reach triple digits speeds is not because of your gears. It's because of your v6. haha after triple digits its about burning as much gas as you can to overcome wind resistance. that's where displacement is needed to keep the car accelerating hard. you can do tricks like turbos and prochargers, gears and exhaust mods, but after 100 mph its about how much gas you can burn per second to overcome the wind resistance that is increasing exponentially.
Sorry dude, but that is totally wrong.
The 2011+ Mustang V6 has plenty of power to top 150.
With an automatic though, the better mod than 3.73's would have been a high stall torque converter, then the 2.73's would allow top speed in third gear.
To the OP, what tune are you using to adjust for the 3.73's???