I'm actually quite good at transposing by any arbitrary interval (by ear), and can also play music both from sight reading and from ear. The problem is I have both an instinctive link between sounds and fingerings (or keys on a piano when I play that) and between the notes in the score and the keys they map to on the instrument. Alas I didn't encounter c clefs until rather late in my musical training (they weren't relevant for any of the instruments I played) and by that time I didn't have a compelling reason to practice reading scores. I'm adulthood I more or less only play solo piano so transposition is a moot point.