In Free Practice, it starts dropping below the second tick on lap 17. That seems to be the earliest point that IMs will be faster than Ws.
While the track is that wet, there is practically no tire wear. It is only once the track starts drying out that the W and IM tires start wearing. But in Free Practice, there is never any tire wear, so there's no good way to test how long the treads will last.
FWIW, the level only dropped to the first tick for me around lap 30.
I remember going IMs in my race for a couple of reasons.
Advantages:
1) If you take slicks, the computer drives SO SLOW on pit exit (which is still wet of course). It's brutal.
2) Of course IMs have better pace until the track really dries out and get off the dying Wets sooner.
3) Yea, the RS runners will have much better pace in the last X amount of laps, but if they want to pass you can hold the dry line and they'll have to go onto the wet line with is insta-death for them.
4) High probability some will spin off and crash while on slicks for some free spots.
Disadvantages:
1) The RS tire is seconds (?) faster per lap once the track dries out. You will be a sitting duck. Contrary to #3 above, I got out of someone's way because I was worried about getting hit he was so much faster. If the slick runners are braking 50m+ later they may punt you pseudo-by accident.
2) IMs start dying really quickly once the track dries out.
3) Contrary to #3 above again, I was having to drive off-line to keep the IMs from going full red before race end (this was in the Ferrari keep in mind which is horrid on tires).
I remember doing this race twice, once in the EMEA (A+) and once in the US (A+) and I
think I went IMs both times (definitely did on the US race).
All that being said I don't recall what position I came out of the pits vs finishing so I am not sure if it was positions gained or lost getting the IMs, although I am 95% sure it was gained. I qualified Q8 and finished P5 (I don't recall which place I started in EMEA but finished P2) but I specifically recall some RS runners fighting late and spinning out. I hate to rely on other's mistakes but that is definitely part of the thought process on crazy races like this. Could be one of those if you think you'll come out in traffic do the IMs; if you think you'll come out on a clear track, wait some more laps and do the RS.
YMMV.