Nobody's been able to get it done. There was a post here a few years ago, and because apparently some of the connectivity with the unit (or maybe it's a CANBUS issue), the 2015 radio wouldn't work in the 2014. The 2015 also uses a completely different OS and I believe some different hardware. Like, the 2015 supports the dual-pane apps (where you can have navi and something else showing at once, for example) and I believe has Wi-Fi, whereas the 2014 doesn't.
Your absolute best bet is going aftermarket. You may be able to custom install an aftermarket unit to go physically in front of the factory one, and use the factory system in aux input mode and run RCA to 3.5mm from the aftermarket unit down to it. Otherwise, you'll be doing what I did and getting the proper wiring kits/adapters (like the SPDIF converter), dash kit, and the climate control unit with it's own LCD display (vs just the buttons that display over the factory screen via CANBUS).
I've done plenty of headunit installs, and the Optima SX-T wasn't too much more difficult, as long as you keep your wiring straight and follow instructions.