I was an avid NR2003 player, from early 2003 all the way until probably around 2010ish, then gradually just sort of moved on to other sims and didn't play NR2003 much. I actually did try re-installing it about a year or two ago but the graphics (even with dx9 etc.) and physics just don't hold up like they used to compared to newer sims. Still a great sim but just a bit too dated for me to really enjoy anymore
Physics aren't IMO at least worse than they are in these modern simcades and that also makes it still overall the best single player Nascar experience, even if not counting the plethora of mods available for it. I haven't tried iRacing SP, but although it is most likely far superior for vehicle dynamics, it is still much more limited for SP and is just for single races. It is also quite costly option for SP.
For nr2003 one of the most important thing is getting the FFB right. As ingame adjustments are limited, you need to do the fine tuning on the control panel. When FFB is set up right, the engine gives really good information through steering wheel for oval racing as the FFB strength changes for the turns. The point is that to get most out of it, you most likely need to fiddle with spring and damping settings on CP. At least I did.
It is quite expected that the games physics engine is not anymore up to current standards, but I am really not that bothered with it because it isn't bad either. Heck, I still enjoy playing GP Legends too, so nr2003 is not a problem. I play even older racing games than that, but I'am a retro gamer and computing hobbyist, so there is that. But I can also enjoy well made arcade racers, so I am not bothered about the "authenticity", but the quality overall. These are all just games and pretty far from the real thing after all.
I still think that nr2003 is overall the best SP Nascar game and is so by quite a large margin. Not forgetting that it still has active leagues going on. Yes, it is not the best racing sim by vehicle physics, but overall still an amazing product. It will be interesting to see what modding community can do with the physics editor, but it is awesome that a game that was released in february 2003 is still alive and kicking well.
Edit: Here are few edited physics available for nr2003 for those interested:
The most realistic physics simulation for NR2003 ever created.
nr2003racingextremes.weebly.com
They can be found and downloaded under series tab. Using physics editor is still pretty much sorcery as many variables are undocumented and thus unknown for modders, but it certainly is interesting. Legendary nr2003 modder Brian Ring (KneePitGaming) is also back and fiddling with the editor.