I do think that someone that is at the higher levels of I-racing could transition to setting up stock cars pretty quickly. I believe its that accurate of a simulation. I just butt heads with them when it comes to kart setups
Some people take what they learn from sim racing too literally. In that case, yes you are going to find it hard to transition. Again, if you use a sim as a tool to refine your analytical process for going faster, rather than specific outcomes of changes from cause and effect (x change in roll bar will mean y result) you will do well.
there is also the one where they took worlds greatest sim racer (gregor huutu) and put him in a star mazda to see how it would go. Turns in fast times but then got motion sick. check it out https://www.youtube.com/watch?v=0p_sCrM1CcI
Iracing has done a very good job since they made the later NASCAR games in the early 2000, even those were used for training tools. You'd be shocked at how realistic they are. Translating that to karting, not so much. Strategy/endurance and how to handle situations do translate
Situational awareness and observation
Focus
Speed sensitivity & trying different lines
Importance of keeping momentum up
Track\racing etiquette
Passing technique
Understanding how to leverage data logging to go faster
I think your looking at it from the driving standpoint. You would be amazed at how accurate it is from a setup point of view. I think a higher level I-racer could setup a late model stock car