Shioda did have a limited time with the Founder as a teacher. Here Shidoa can be considered similar to a student who theoretically the Founder withheld information. Shioda's skill didn't deteriorate, it improved greatly. Here we are looking at the benefits and development of a student who lacked much information and still developed great skill. Was he better or as good as the Founder? Good question, I don't know. It is evident he did improve with limited knowledge and instruction from the Founder. It answers the question for me, the Founder didn't with hold information.
Shioda was actually with the Founder longer than almost anybody (except maybe Saito and Kisshomaru) - at a time when the Founder was very active.
As to how well he did - I'd suggest you look into Shioda's relationship with Kodo Horikawa.