Interesting post from Bye. The slippery film he describes makes me think of the effect we get from de-icing fluid.
The most I have noticed the effect of rain in over thirty years of gliding was in the Libelle. A beautiful early glass glider which really did not like wet wings. The rate of sink increased markedly.
On the Boeing I fly there is no noticeable difference. However a basic understanding of practical aerodynamics suggests any contamination must effect performance. However the difference between a glider where you may at times be only few knots over the stall speed and a jet where at the slowest point you should be some twenty to thirty knots over the stall speed mean you do not notice it.
The only time it is an issue is with flaps 15 vref ice. I understand Boeing discovered in testing that the margin was reduced in this configuration which is the reason for the add on. Presumably one could expect a similar but lesser effect from rain.