The short answer to your question "have they fixed the transmission issues for the 2015 model?" is...
NO THEY HAVE NOT!!!
Here are the issues I've experienced in the almost 15k miles I've owned my fully loaded 2015 FiST:
The first things I noticed were slight rattles from the interior as well as interior trim not fitting correctly (huge gaps between the center screen and dash etc.) and usual tech-gremlins (my sync system is a joke) I just accepted all of that as part of buying an econo-box turned rally car.
The second thing I noticed was a clunk when shifting between gears. Took the car to the dealer. They replaced the top right motor mount and whala, the noise was gone. At this point the car had less than 5k miles. I was happy with the service departments solution, but concerned with the quality of the assembly of my particular car.
The third thing I noticed were paint chips. Yes the paint is really thin and chips easily, but the paint on my FiST also started peeling off in large sections after it chipped. I didn't accept that as a part of buying an econo-box turned rally car. Eventually, Ford repainted my front bumper under warranty. The body shop they sent it to... well... When I picked my car up from the dealer there was a !warning! in the my ford touch system that said the car was stalled at 9:45pm (when my car was supposed to be in the shop). The service department at the dealer called the body shop; the body shop denied everything; at which point the service department wiped their hands of it and claimed they'd "done what they could" (not their car, not their problem).
The fourth things I noticed were that the black vinyl/sticker trim around the windows started bubbling up and the interior trim inside the small triangular A-Pillar windows started to warp. Also, the seat warmers on my $2,000 Recaro's fluctuate between very hot and basically off every 10 minutes (Ford claims this is also a "normal characteristic of the vehicle"). Again, I just accepted those were a part of buying an econo-box turned rally car.
The fifth thing I noticed was a rattling noise when shifting between gears. I assumed the top right motor mount was bad again. However, the noise grew, and grew, and grew, to the point that it was very loud and present when the car was sitting still and idling. The noise would go away when I depressed the clutch pedal and return when I let the clutch pedal back up. I dropped it off at the dealership and picked it up a week later. The dealership called my issue "engine rollover noise" and a "normal characteristic of the vehicle." I didn't accept that and took my FiST to another local Ford dealer, who determined the noise to be either a throw-out-bearing issue or a clutch issue. I know it's not the clutch because the clutch has never slipped and has never been burned (unless the body shop that repainted the front bumper took my car for a joy ride and stalled it at quarter till 10pm when they were supposed to be closed and my car was supposed to be in their shop, but again "not Ford's problem or liability").
I stopped by the dealer today because the service adviser hasn't been keeping me in the loop. I learned that they hadn't even started on my car, it's just been sitting in the lot, waiting for a lift to open.
MY FIRST 14,730 MILES HAVE NOT BEEN GOOD! I REGRET BUYING MY FIESTA! Which is a shame because I was really excited about the Focus RS and planned on buying one. After this, I think I'll just save up for a performance car that's not such a "bargain." I definitely learned that in life (especially with Ford) you get what you pay for.