I'm not sure he personally studied them, or just discussed the previous work. eg. from his '93 paper discussing the violinists:

It is important to note that our study shows only that the amount and distribution of practice is related to the level of performance of adult musicians. In fact, many additional factors consistent with the skill-acquisition framework could attenuate the differences among our three groups. Sosniak (1985) found that international-level pianists had spent considerable efforts to seek out the very best musical teachers during their musical development. Furthermore, it is likely that an analysis of the detailed activities during practice alone would reveal qualitative differences between violinists at different advanced levels of performance (Gruson, 1988; Miklaszewski, 1989).