Hi Liz,
you had a great idea about comparing root sizes from different suppliers and next few year development online, so we will find out in 2 or 3 years!
Personally I would rather divide the roots right away when purchased or transplanted than to wait another few years and use enormous effort to dig and divide them. IMHO best transplants are with 4-6 eyes and nicely developed root system ( 4-10 medium sized spreading roots - something similar to my divided Red Charm or first division of Florence Nicholls). Praying to peony gods helps sometimes as well.
