Rain is something they have previously done and done well - talking about physics of course.
I'm not worried about them doing a decent job of it - just that they've repeatedly said that to do non-dry surfaces to the same fidelity compared to the real thing as AC is to dry is really hard and to put in an approximation, even a very good approximation, would be hard to do within the same model while not compromising the fidelity of the simulation on the dry (i.e. due to handling transitions from wet to dry areas and vice versa, etc.) - which to me kind of indicates combined with the engine change (at least for rendering) that it probably will be backed off a little from AC.
I'm not that concerned about it as such as I think they will do a good job and hyper realism or trying to reproduce the illusion of it in a game or simulation isn't always a good thing even in a simulation (of this nature) - plus GT3 cars tend to have ridiculous handling anyhow.