Actually the reason the cars feel like they gain traction is for several reasons. One Ninja explained is that when the road starts getting wet all that rubber in the track starts coming to the surface when it starts raining. The second thing is a obvious one. Is that the comfort tires have deep treads in them. So water flow is much higher than other tires. So when it rains grip is much higher. GT6 actually improved this over GT5 in terms of Comfort tires in rainy conditions.
But I still think its gonna be a fun round. I love rain drifting. I try to tune all my cars to be able to run in all sorts of conditions. So I spend alot of time practicing drifting in the rain.