First of all, F1 cars bodywork is not as rigid as you might imagine. to preserve the car from vibrations and to allow aero loadign to be passed through to the chassis it is reqiured to have a certain level of stiffness. But weight saving is just as important and bits do flex, seeign the cars over kerbs, or if you have ever seen a car on a seven post rig, everying moves about quite alarmingly. So viewers may see winglets wobble, front wings shake side-to-side and flip ups rattle, this is normal and not a sign of design deficiency.
But I have witnessed form the onboard footage and potenitally performance enhancing flex on the williams front wing, that I have not seen elsewhere.
I think there are two different cases of wing flex beign discussed here. From my view of the on car footage the front wing does flex but is longitudenal flex and not horizontal flex. From the sketch of the onboard picture, I see the flap (the tilted part with "Fed Ex" on ) flex reducing its angle of attack to the oncoming flow (shown as A on the diagram).
The flap flex that I beleive occurs is designed in and as it is not overed specifically by the rules cuodl be used for circumventing the rules. As the flap is the part subject to high loadings it can bend backwards reducing its pitch and preventing greater amounts of downforce\drag being created at higher speeds.
I have heard discussions about the main plane (the flat part with "HP" on) shown as B on the diagram, I have not seen this part flex, this was a strategy used a few years ago to make the endplate closer to the ground to improve sealing and move the main plane closer to the gorund for increased ground effect. this latter type of flex was prevented by the Scrutineers placign a heavy load on the endplate and measuring its deflection. No doubt there is still a dregree of flex in this part, which is designed in. However the reason for designing this flex is not to cheat, but to make the part only as heavy as it needs in order to be stiff enough to meet the loads and scrutineers requirements. The small resulting small amount of deflection is not a performance aid.