My experience with nitrogen purging-
I have 5.5" width rims fitted to my Swift Vxi. The tires were 2500km old and car 3 months old at the time. My tires are 165/80. The fitter also nitrogen purged my tires. Immediately after filling, I took the car on the highway and I found it to be more planted, though the ride had become harsh again. I had been filling air at 33psi initially as recommended by the MASS and the manual but after complaints on the harsh ride, I dropped down to 31-32 psi. So when the fitter fitted alloys and purged nitrogen, I suggested he maintain the same pressure. He said that the car should have 35psi in the front to carry the engine weight and 34psi in the rear. That's what was causing the harsh ride, so I vented the nitrogen down to 33 in read and 32 or something in the front. This added ride comfort without unsettling the car
After then, I didn't check pressure for 2-3 weeks. When I checked the pressure at an air pump after 20days, it was still 32psi. Now I keep 32psi without re-topping off nitrogen, just use air
Considering air has 72% nitrogen, it shouldn't make that much difference in the ride if nitrogen is purged. Specifically, I find only marginal improvement
Ameyam