For real practice get iRacing. I know you run at Road Atlanta a lot so it would be incredibly beneficial for you.
The tracks are laser scanned and VERY accurate. From what I can tell VIR in iRacing is spot on with VIR in real life.
The good part about iRacing is they run pricing specials often and you only need to buy the content that you want to use. Though I happen to be one of the fools in the 100% club.
The sim in general is just fantastic. The tire model is very realistic and like the tracks the cars are all torn down, scanned, and re-built in digital form.
I actually won't use anything else other than iRacing because it's too arcadish in comparison, granted I've been a diehard sim racer since the mid 90s.