To OP:
Well there are three separate issues.
1)
One was aftermarket wheels on MR cars. If installed on an MR car, the wheels would become the same width in the game visually and physically.
That would cause very bad handling and make them impossible to drive aggressively. I noticed this first thing without ever hearing about it in the forums when I fitted aftermarket wheels to an NSX early in career mode. I didn't realize it was the wheels that were the problem until I read it here though, I didn't make the connection and just thought MR handling was screwed on that car. After that, and well before 1.03, I returned all MR cars' wheels to standard and everything was "ok" for me, except on Audi race cars for example. More on that below in "3)" edit- This is fixed. I tested it.
2)
The other issue is something about online races and S spec career race pit stops where on the second set of tires the rear tires would wear faster than the front and also the rear grip would be gone, causing slower lap times, but only seemed to affect MR cars regardless of aftermarket wheels or not. This apparently only effects racing tires.
I noticed this yesterday when doing an S spec career race at the Ring -24hr track in a Stealth NSX race car. I then read about it here and made the connection before the patch came out. Have not re-created that scenario yet to see if it was improved but there is a thread about it
here:
https://www.gtplanet.net/forum/threads/gt6-tyre-bug-on-mr-fr-cars.293953/
and here:
https://www.gtplanet.net/forum/threads/weird-bug-after-pit-stops-in-online-mode.294753/
Apparently this is not fixed going by reports.
3)
The other issue is what I have noticed in GT6 myself.. and have heared it hinted at in various posts. It's that new MR cars developed after GT5, not ported from GT5 to GT6, have some inherent problems with grip and handling compared to the MR cars ported over from GT5. This is not fully tested, and just an observation that may have some exceptions.