I have a '16 with factory 19" wheels and reasonably complaint tires (Michelin Primacy MXM4s). Even so, the ride is too brittle for my tastes. It rides less comfortably than my old G35S. I've played with pressures, and that doesn't make much of a difference. I'm sure some of it has to do with the suspension, but still. It's not great.
Has anybody downsized to a 17" or 18" wheel? If so, did the extra .5-1" of sidewall make a noticeable difference? I've never driven a QX50 with the 18" wheels, so I have nothing to compare to. I'd go with something used from Infiniti, likely off of a Q50.