RE: Fold down GoPro on bottom of fuselage
Welcome to the site, skibby !!!
If you refer to the changes of angle of the camera during flight acting as a control surface, I believe that the worst case is the view straight ahead due to maximum drag.
However, for the low speed of cubs, it should be not too important as to disturb flight much.
Yes, it will be drag inducing some nose down pitch at high speeds, but that could be easily cancelled with elevator input or trim, IMHO.