Hey GT-11, I like the physics on both GT-5 and iRacing, but I do feel that iRacing makes you use more realistic inputs with respect to how gingerly you need to drive a real car; in order to be smooth and maintain speed. My biggest gripe with GT is the horrible tire model; I think each compound feels good, but the wear is just dumb. Dumb. Dumb. Dumb.
The only complaint that I've had with iRacing and will probably have later this week is the over sensitively in the MX-5 and mid engine cars like the SRF. I feel they are too prone to oversteer, but I know can be mastered with time; I just hadn't given the SRF any real time, and I only found the MX-5 super twitchy after the new tire model update. I also found that I had trouble in it after racing the Mustang for an entire season then switching back; I had a hard time to adapt back to it. I know others have had the same issue when getting "very familiar" with once car vs. another. This is another thing I don't notice in GT5 where you can typically jump from car to car; in iRacing I don't find it that easy.
The biggest difference of comparing tracks from iRacing to GT5 is Spa, because Spa was one of their most recent builds it is night and day vs. GT5. I think Laguna was one of iRacing's first builds and hasn't been updated in a while.
Stepper - it took me weeks if not months to get my setup exactly where I liked it. There was a lot of work in balancing frame rate vs. graphics quality - field of vision (I only use 1 screen) - and wheel and pedal settings. I haven't experienced any headaches or issues with iRacing vs. GT like you are experiencing.
Also, I know that you guys are comparing times and one neat feature of iRacing is it logs all your fast laps for official practices, races, qualifying, etc. The key to this is you have to join an "official" practice session for your laps to register, it won't happen in testing mode. I'd always recommend to practice in practice rooms rather then testing solo because other people will be good benchmarks plus many times they'll share setups (which is a great feature) which you can plug in and try immediately.
Sorry for rambling on!
Cheers!