More to do with the only model in the range with a sunroof has 18" wheels. Just traded in my second i40, both were premium with 18" wheels. I thought both of them were fine and quite comfortable. I had one of the original 2011 model year cars and put 50k motorway miles on in 18 months and it was spot on. The one just gone, a 2015 revised model was slightly more refined in some areas but no different in terms of handling.

So traded my VW CC private hire taxi in and now back to Hyundai i40 - my 3rd. My question is would 18 inch wheels make a big difference in handling from 16"? The ride is positively worse than my previous 2 that were on the standard wheels. I have also been told it's because I have a glass roof! Really??

Unless already done so, I'd check to see if its even possible to downsize by 2 inches, as some cars have larger brakes and cannot accept smaller alloys. I just changed my Mazda3 mk1 (1.6 petrol TS2, built late 2005) from 16in to 15in wheels and tyres, and its made the ride SO much better, ever accounting for the new tyres. If I'd owned a 2.0 version (even if with the same TS2 spec) I wouldn't have been able to do that as the brakes on that car are 16in instead of 15in on mine.

If you stick to decent quality tyres, I doubt you'll notice much difference in handling abilities (especially as taxis are exactly thrashed like a track day car - they lose a lot of business if they did), except in snow and ice, which will be improved.

I also changed my tyres from (decent) Dunlop summer tyres to Michelin CrossClimate all-seasons (Goodyear ones would be fine too, especially if you lived up North) which would help in winter and still give good traction/handling at other times as well as a more comfortable ride. Admitedly they would wear a bit quicker than standard summer tyres and are more expensive, which for a taxi driver like you is quite important.