Interesting that you have found no difference with 17" tires. Two explanations. 1. the tires are also stiff (again, brand/model makes a difference here which you haven't noted). 2. Your offroad comfort needs exceed the engineered suspension travel and capability of the NX.

If you're convinced the tires won't make a difference, personally, I'm fairly convinced point "2" is your situation and that changing the springs and/or shock absorbers will also make very little difference. At this point, you should find a non-F Sport and test drive it before taking your chances on suspension parts.

 

You're so welcome! From what I could find, 225/65R17. The 17" rims are 5x114.3
http://www.wheel-size.com/size/lexus/nx/2015/

If you want to keep your current Rims and save some $$$, you might be able to go with a slightly taller & beefier tire to absorb some of the road bumps. Something in a 225/65-70R18 might work. I've seen people put 20s on the NX, so It can accommodate a larger wheel/tire setup. Just make sure that it doesn't rub, watch your pressure, and what not. I don't think I'd personally recommend doing it that way though...