Ok...i did the following....
Set the track to rain (non fixed). It started raining immediately. During warm up, I got about 2 laps in with overcast sky and heavy rain, then it stopped. On lap 3 i switched to slicks to see if they would grip on the wet, but not raining track. They dont. I had to shift at about 2,000 rpm to keep the torque from breaking loose the tires. 30 mph through most turns, 60 mph on the straights. I ran another lap with intermediate rain tires and that was much better.
Started the 10 lap race. Track was just how i left it in qualifying, wet but not raining. On intermediate tires, i put down a best lap of 2:40. The track remained wet for all 10 laps but it did not rain again. Lap 5 i switched to rain tires to see how they gripped on the wet track. I put down a 1:38 but tire wear was much worse. I spun out once or twice but after only 2 laps there was noticable tire wear on my rear tires. Lap 10 i pitted one more time for racing hard tires. The track was still damp but they gripped just fine.
Conclusions....
1) even if it isnt actively raining, you still may need rain or intermediate tires
2) rain tires grip a little better than intermediate tires when it isnt actively raining, but they appear to wear a lot faster.
3) at some point after raining, it will be safe to switch to slicks...but it is unclear how soon. I essentially changed over 13 laps after the rain stopped but i might have been able to switch over sooner. You cant use tire mist to judge, since i still saw some on my final lap.