I have a 2017 GT Line S and when I was checking the fuel economy brim to brim I was getting about 43-45. My main driving was/is to our caravan, a round trip of 110 miles and to football matches, we don't do much town driving we use the wife's picanto for that. I use the optima in eco mode.Good Lord! I must have missed that post but had I read that, like you, I would have had grave concerns.
There is a handful of 1.6 litre owners in the forum and 'Tommo21' springs to mind as an occasional contributor. If he reads your post, he may well be able to confirm that you will probably average in excess of 40mpg unless you are particularly heavy-footed. When I say average, I mean exactly that as it is pointless to claim 50, 60, 70 or whatever mpg the instruments display on a steady speed A-road or motorway trip in light traffic in midsummer.
Although the 1.6 engine is slightly 'greener' than the older 1.7, I understand that the performance and economy are broadly similar in practice. In my hands, since new, mine has returned 43mpg in round numbers over the piece and that includes quite a bit of heavy load-carrying on a fairly regular basis - that's what estate cars are for!
You may wish to play around with the drive settings but I was happy to settle for the regular, normal setting after some experimentation with mine. Any advantage of the Sport setting is spoiled by the increased engine noise in the cabin due to the DCT frequently being in a lower gear than necessary with higher engine RPM. ECO in mine may save a pint of fuel every 1,000 miles 🙄 but it dulls the accelerator response and I prefer it the way I have it set up.
The suspension is quite firm on the GT-Line S but rewards the driver with good handling through bends with little body roll, even with the top-heavy body, due to the panoramic sunroof.
Those are just some of my thoughts and having owned quite a few estate cars over the years, all things considered, I put KIA right up there with the best of them. It isn't the quickest by any means; it isn't the most comfortable but it has proved extremely reliable with decent fuel economy, allied to a relaxing drive. I hope you are not disappointed.