You will miss the Fiesta. Yeah, the mustang is faster, in a straight line. What the Mustang isn't is nimble like the Fiesta and it's not close to the fuel squeezer the Fiesta is.
I also have to ask if you have nearly 500 hp on tap with massive torque spread why is a freaking 10 speed transmission needed. It is something I really dislike about cars today, things that should target simplicity and durability have become complex Squared because people think more gears just have to be better. Think about it, used to be a 5 speed was plenty good enough and the only real benefit to a 10 speed is that you get to pay megabucks to have your transmission replaced with a new one because the service techs today are not qualified to repair a tranny, they are only qualified to swap it out. I can remember helping my dad with a Saginaw 4 speed back when I was only 12 years old and it was actually pretty simple to fix.
BTW, did time driving a semi for my dad while home from college for the summer and with those trucks 10 speeds did make sense. Because the Cummins 350H pulled best between 1800 and 2100 rpm, so when there was a load of 40,000 lbs behind you 10 speeds allowed you do get up to speed and carry more speed on the hills. However when running back to base empty we didn't bother with split shifting because an empty trailer was easy to get moving.