Can I suggest deferring any discussion about whether X player studied theory or not, and focus on providing information and experience of theory courses and exams.
I completed grade 8 ABRSM theory many years ago and found it a wholly enlightening experience, and my playing of any instrument (both in terms of skill and enjoyment) have increased as a result. The ABRSM workbooks and sample papers are cheap and excellent. My 9 year old recently did the grade 1 exam, scoring 98%! It has reactivated his interest in the piano.
I'd say do it, and I thoroughly recommend the Associated Board. Get a teacher if you need one, but read the books and do the sample papers yourself first. And feel free to PM me with any questions.
Martin.