Here's a great read on the topic: Sarcoplasmic Hypertrophy: The Bros Were Probably Right
I thing the OP question is whether we can influence the percentages through different training methods or if it's predetermined genetically. And even if ratio of myofibrillar to sacroplasmic hyperthrophy is not fixed then how much realistically it can be changed? Is that even significant? If difference in ratios between doing 20reps and triples are within statistical error margin then it's not something anybody should be concerned with.
PS. Even in PP there's a mention about higher rep, higher volume work promoting more "sarcoplasmic hyperthrophy" but without getting into any details.
I think it's pretty clear that sarcoplasmic hypertrophy will be greater when the sarcoplasm has to expand as a response to it being insulted during training. If it is not stressed, it will not increase to the maximum amount. That said, programs designed to drive hypertrophy optimally can do this without sets of 10-15, but it's not something anyone should worry about unless they're worried about getting too big or too strong.
I was rather interested in knowing if strength being a function of cross-section area of muscle, power-lifters out-squatting bodybuilders is due to factors other than training and practice. Also, it's partly related to this old post .