Good question. I was doing some research because I have some 245 40s sitting in my garage from another car. Evidently they may fit, may bulge a bit, may increase the diameter around 2%.
Safely, on that size rim, 235 will fit by most all manufacturers. Another 10mm may be possible, because manufacturer's sizes vary and one's "235" isn't necessary the same as the next "235", but that kind of depends on the end result. If you want to go that wide, you should pick up some wider rims most likely. Mine will come with the crappy AS tires due to the DCT, so that makes me want to try the 245s I have to see if they'll fit, but if you are rocking the AS tires, you'll get a huge improvement by going to PS4S 235s, probably a lot more cost-effective than going a little wider with a new rim. I have an excel program to calculate profile (ratio) and height, so I haven't run these through that yet, as in what profile would be optimal with 235, it's likely slightly lower ratio will be better. Offset and inner rubbing is the other concern and I don't know if my tires will fit on the font with this in mind. Again, I have a calc for offset, but I haven't plugged any numbers from my coming GT N-line in it yet.
One way to get started on this would be to find out the offset of the stock wheels on the N-line and compare those to the offset of the i30N. Once I have those data, I can figure out what will and won't fit on the inside edge most likely.
Curious to hear if anyone else knows. Maybe ask on the N-cars.net website, seems to be a lot more traffic there.