I've been doing some testing as well. All of my times are 3W, 2RM. This is the safest option. Of course, you can push it and start on IMs and/or pit for RMs at the end of lap two, but in my experience you're really chancing it. Doing this in TT is one thing, when you have the whole track to yourself; but, in a race with other cars you can't always be on the racing line (which is the driest part of the track of course). If you may recall, during the last Manu race at a wet Nord, people were dying on the Dottenger just from hydroplaning. Also, hitting the brakes can get quite sketchy on the more aggressive of compounds.
BUT, for lap two and three, the track is in IM range and half of lap three is virtually completely dry which will eat those W tires to shreds. I did a Custom race and did one dry lap on the W tires in the Mustang (with 2x tire wear because that's what it says in game despite the Site saying 1x tire wear) and it at the Ws about 50%.
I've tried a couple times in various cars to start on IMs but have yet to make it through a lap without spinning or hitting the wall. Sometimes, everything is fine and I'll hit a puddle or something I can't see and it's just gone. I feel like this could be the same problem trying to pit for RMs at the end of lap two (as
@Moose78 mentioned). All it takes is ONE of those instances in a full lap around N24 and you're done (especially in GT1 with heavy damage). The Mustang, for example, spins the rears while on full wets on the straights in 4th gear, with TC on (hence why the time on lap one is so slow)!
So it's a bit of a handful and would likely die on IMs pretty quickly.
Pit delta seems to be ~28 seconds, so a two stopper is likely not an option.
Here are my times thus far, starting from left to right. You can see the times get better but that is due to the extra practice. The Porsche is capable of 8:0X for me I think on the RM dry lap. I haven't done a five lap stint in the McLaren yet. Hoping to try some of the same theories as Moose, doing 3x IMs, 2x RMs, or 2x W to 3x RMs, etc. and see how they shake out.
So do you go aggressive and try for the lower lap times with a higher probability of crashing out (especially in a race), or do you sacrifice time to be safe? THis is shaping out to be pretty exciting as I think a lot of people will be running different strats.