Generally speaking the car with higher downforce conferred by the aerodynamics of the bodywork and wings will have the advantage in wet and changeable conditions. The ability to minimise understeer and to maximise traction is more aero dependent in the wet. Mechanical grip is most vulnerable in the wet since both ground effect and the contact patch of the tyre is reduced. It is said that inclement weather levels the playing field but in reality that is only insofar as a car with less horse power and torque tends to be easier to drive in poor conditions than something more powerful. Since the power differential between the teams isn't realy all that big these days, it's still the car with superior aero' and the superior driver that excels. DRS and KERS are probably a little less useful poor weather due to the potential loss of traction if applied inappropriately.
Taking all that into account I think it safe(ish) to say that recent changes in the rules did squat with regard to wet races. However, changes in the attitude of race control have since we now get far too many laps under the safety car instead of wet racing.
