I have 225/50-18 on mine as well, but I think the speedo is a little bit faster than stock. When I was on 225/45-18 and the car meter was at 80mph, the actual would be 81mph (speed reported from Waze or Google Map). But now, at that same 80mph on car meter, the actual is 83mph.I wanted to reply in case other people are facing the same issue of hard ride: try taller tires, 225/50-18, and you should notice like I did how it feels better in terms of being a softer ride. I don't think you need to go to a 17" rim, just need a bit more height.
Also, there is a possibility that the sportier suspension is what is causing the issue for the passenger, so your best bet is to try the taller tires and hope that is enough to address the concerns, and then if not, try getting softer suspension components. I just feel like swapping the wheels to 17" is too extreme and will be wasted money because it's better to solve the issue by either using 1) taller tires on the same 18" rims, and if that's not enough, try 2) softer suspension and keep the 18" rims and use taller tires.
Note that I really like using 225/50-18 on the 18" rims, and have been using it for the past 2-3 years. I'm about to buy tires again and I plan to stay with the taller tires. I also like how the tires fill the wheel well a bit more, looks nicer than the stock tire size on 18" rims. I think it's a 3% difference, so your speedo is corrected to be more accurate (instead of how the stock size results in the slightly optimistic/inaccurate speeds that is typical for default cars).
What tire do you use? I have a Yokohama set, the tread wear is extremely great, but the road noise is not better than my old set of Michelin at stock spec.