ORIGINAL: OzMo
If Mastering means becoming competent and flying at the limits of the craft then few people really master a trainer. Try some one wheel touch and goes. I also teach my students to do axial rolls and get good at slow and low flying. Inverted under good control is a good measure IMHO also.
That is quite true... people who believe that they have "Mastered" a trainer, quite often have not.
Even an experienced and able pilot can still learn a thing or two from a seemingly whimpy trainer.
( And YES you CAN "Master" a plane, or anything you want. Mastery is nothing more than a high degree of PROFICIENCY. )
We have a few Giant Scale ( 50%!!! ) flyers at our field, who perform some amazing stunts with their big birds.
One of the guys who flies a 50% Yak, pulled out his old trainer and now brings it out to the field with some regularity.
While we all kidded him about reverting back to a trainer, he commented about how it was more difficult for him to do things on this smaller, less acrobatic plane, which made it a bigger challange for him.
He mentioned that he felt that he had mastered his Yak, but he was still working on his trainer.