Folks, its not time to shoot down DP Davies yet, especially as his advice on necessity of utilizing perspective and balance when reflecting on aeronautical matters has been once again proven to be useful - by this thread.
Yes, rain will affect aerodynamic performance of any airfoil.
Yes, the effect was measured in aerotunnels.
Yes, it is significant on laminar aerofoils such as used on high performance gliders or Rutan's canards.
No, there are no modern airliners using laminar wings. Certain Tolouse based brand claims their planes do but while this might not be entirely marketing hype, there are no incidents showing they lose performance or their control forces/efficiency alters in precip. Mind you: some of their make and models are almost tragically underpowered.
No, there are no airliner accidents or incidents where high rate of precipitation was proven to be decisive factor. Even when extreme rain caused both engines to flame out, aeroplanes were controllable down to ground/water contact, outcome largely dependent on the quality of the terrain below. Anyway, there is not much point in worrying about loss of performance due to wet wing if you find yourself in microburst.
Paper quoted that states there were some a) doesn't provide any reference for the claim b) is published via Azerbaijan based pay-to-publish operation that organizes conferences used just to pad the CVs, despite the claim its journals seem not to be peer reviewed at all and consequently has pretty bad reputation in scientific circles.