I have had the same situation, its loses its softness - I haven't driven the car yet I parked it and did the wheel - but it felt softer. I have avoided chemicals so far - only using a damp microfibre and a dab of hand wash, but over the 4 years - it has started to feel less soft. Almost flat to the touch. This may well be a cheap fix to the problem, as I brushed a small amount of debris came off the wheel.
While the Nismo Wheel is great it loses its touch over time,
Current - 2014 (14) 370z NISMO in Storm White #229, JDM Carbon Nismo Kickplates, JDM Carbon Nismo B Pillars, Nismo Tyre Caps, Nismo Oil Cap, K&N Filters, MY15 Nismo Start Button. GT-R Dome Lens Covers - LED Lights (Interior, trunk & numberplate) - Demand Engineering Full Valved Custom Exhaust System. Custom Blank Button Delete Z1 LHP Upgraded Hatch Struts
Previous - 2005 (55) 350z Azure GT Coupe, Parking Sensors, Z Tax Holder, Z Mats, Z Door Guards, White Reflectors, Z Tyre Caps, S2000 Aerial, BT Kit,