The previous place I lived in had very hard water, and I found that running the water through both a coffee filter and then a Brita charcoal filter worked rather well to soften the water and improve the quality of the rice I was cooking.

That's aside from whatever harder-to-measure taste differences it might have, but it makes a big difference by itself.

I would be amazed if rice grown by Japanese immigrants like Koda or Tamkai couldn't reach the level and it really was the land that made the difference.

The only experiment I have found that attempts to measure this is by Kenji with pizza. In his test, the water did not matter.

https://www.seriouseats.com/does-nyc-water-make-a-difference...

Nishiki uses an offshoot of Calrose rice, which is okay but popular Japanese rice is often Koshihikari or Yumipurika: https://en.wikipedia.org/wiki/Nishiki_rice (do click the link for SICK web art on a random wikipedia page)

If you can find some https://umamimart.com/products/tamaki-haiga-rice is one of my favs.

Or as someone commented below the rice from The Rice Factory is solid if you are looking for a treat: https://trf-ny.com/collections/rice

I currently have 3 varieties from TRF in my cupboard :D