I had a race in the Rain last night in Kie's lobby, and the heavy rain is tricky. I tried the GT3 Hyundai out first (thought light weight, n/a engine and low torque would make it drivable but I was wrong) so switched to the Mercedes AMG and it was a lot easier. It is still hard work and I cheated a bit by bumping up the TC to 2 in the race (rear wet tyres apparently wear quickly so I thought the TC would help). Don't think wet lines really exist, but the game is tricky in heavy rain.
Managed to pick up the pace as the race went on so the Merc is worth a try. Porsche is probably the best though, with the BMW Z4 another decent choice (according to Kie, I haven't tried them yet).
Game is fun in the rain and looks good. Very nice update and looking forward to future tracks in the rain.