I've just done a track day in the wet today. I did one a few months back with a poor wheel alignment, and road tyres which had gone hard from being on the track a few times before. It was awful.
Today, with wheel alignment settings as suggested by TitaniumDave, and on Toyo R888s the car was great. I'll post some video when I hae edited it up a bit. Now when I mean rain, I mean rain, not the slightly moist pictures posted above. The only nervous moments were going through the full track width, pools of standing water.