Couple of observations so far:
1. Track lighting is different. Go to arcade mode and set the time to 7.30. In arcade the sun appears "whiter" while in the GTA event it appears more "yellow/orange". Probably PD testing GT7 lighting?
2. Car setup seems a bit different from last year. A lot more lift off oversteer this time around. Good change from the 1st round! Also this means hopefully no handbrake for the top times as the normal brakes are oversteery enough!
3. Different track grip levels are back again

I struggled to get below 1.25 after around 10 laps on my first entry. Quit the event and re-enter and instantly 1.24.7 on the first lap, then 1.24.3 on second lap. PD better fix this crap for the final round or it will be a complete farce.
4. Cone placement is killer, especially for the three 90 degree right handers after the back straight. I'm normally very inconsistent for those corners too so it will be a lot of retries for me
5. Lots of late apex corners. Slow in fast out is the mantra here. Short shifting early also helps. The big hairpin before the back straight is CRUCIAL!!! (sadly another corner I'm really bad at

)
The track grip level varies randomly per session. So you can have very little grip, but if you quit the event and come back in you'll have heaps more grip and be able to set a faster laptime a lot easier.
Looking at the first round, the final top time is around 1 second faster than the top time of the first day, so yes 1.21 flat is probably the limit. IMMORTAL could probably get high 1.20s if he pushes hard.
As always, good luck to everyone!