RX-400h has been my dream car for a long time.
RX-400h was quite a little bit bumpy if compared with ES350
RX is wheels are 235/18/55 and the ride of ES(with 215/17/55tires) was smoother. (pretty comfortible)
l took RX once in Tailand, the ride was just smoother than that of the RX today. l am not sure the RX in thailand had thicker tires than the one we testdrove today. l doubt those tires were thicker in thailand.
Rx is suspension is harder. If l change it from 235/18/55 to 235/17/65, the ride of RX will improve a lot? is the thickness of 55 just too thick for SUV? After the test drive, My parents prefer ES350. l liked RX-400h but l was disappointed with the bumpy ride on RX400h. So, l think putting on thicker tires will probably improve the ride quality.

The ES350 is supposed to be a smoother riding vehicle, it is a passenger vehicle, while the RX400 is an SUV, designed to be used on multiple terrains while being stable & safe. Different tires may make a slight improvement, but will not change the overall handling characteristics of the vehicle. Another consideration to keep in mind is service & maintenance costs & availability. The Hybrid is a great vehicle, but finding good quality service will be harder, especially in remote areas.