Not sure hardiness is the issue,usually vigour is one of the reasons for grafting.However sometimes it's good practice to use less vigorous rootstocks with certain cultivars(though often not practiced.)
Not sure if it's entirely relevant but one of our largest maple propagators here uses 'Senkaki' as their rootstock for a lot of their maples.Wether the offspring of that cultivar should be any stronger than a parent on it's own roots I'm not sure,but I don't necessarily see why they should .

Interesting... I may keep this one in a pot over a couple growing seasons and see how vigorous it is. Unfortunately, I don't have a (same cultivar) baseline to measure against. I am growing two other maples ('Autumn Moon' and 'Orange Dream') in net pots and 'Orangeola' and 'Acontifolium' in ground.

Thanks for the input

John

Very wise John,apart from perhaps being on more vigorous roots,grafted plants have another advantage I think is often overlooked.The 2/3year graft you buy will have 4/5year old roots,no-wonder they do better.
I have Seiyru,Katsura and Beni Komachi on their own roots,though still in pots they are probably big enough to be planted here.Patience will be your greatest challenge,good luck.