I wouldn’t be taking the car to a transmission specialist…yet. Since the car is still under warranty, you need to give Kia every chance to sort it out for you. The good news is that this won’t cost you a cent, whereas taking it to an independent repairer will start emptying your wallet.
You’re doing the right thing by keeping a close watch on the transmission temperature readings (provided that’s not at the expense of watching the road) and if overheating was what caused the warning light to illuminate, then I’d be of the view that the transmission fluid should be changed (at the very least) and an investigation carried out into why the transmission overheated in the first place.
I haven’t heard of a rash of problems with the Rio GT-Line’s seven-speed DCT transmission in Australia, but Kia’s DCT gearboxes in other models have been known to give problems in other countries. Given our typical summer ambient temperatures, we probably won’t be immune from the same problems. This is not a Kia thing, by the way, but seems to be one of many problems that have plagued DCT (double-clutch) gearboxes over the years from a variety of car-makers.
The clutch packs in these gearboxes have to slip to work and that’s a well-known cause of heat in any transmission. If they slip enough, they could conceivably cause the transmission to overheat at which point it will trigger the warning light you’re seeing as well as not operating properly and potentially doing internal damage. Both Ford and Volkswagen have discovered this the hard way, replacing many double-clutch transmissions under warranty over the years.
It’s a bit hard to see why so many manufacturers are persisting with this technology, especially when the modern, multi-ratio conventional automatic is so compact, light and efficient these days.