I trialed B. praemorsa last year and moderately cut back the roots in Summer. They powered on and did well initially.
But eventually they both died within 8 months of the repot. Hard to say why they died.
It was disappointing as I liked the whorls of twisting growth that mine developed.
This is just my own meanderings, but I feel there are Banksia that are quite hardy, and those that are just temperamental with bonsai conditions. I never found stock that was inbetween. With Banksia species it seems you can have really hardy material, or really painful material.
But more likely its just that the species is better suited to certain areas I presume.
Because with root prunes, I have never had any great issues removing roots from Banksia stock. Its normally not enough sun or needing fast drainage that can be a problem for a lot of Banksia.
Touching on what Shibui said in a different post, ... a lot of the dwarf Banksia would really suffer if you didn't have good sun for a good month after repotting for the first time. That certainly was a factor that I noticed many moons ago with Banksia varieties.
The new foliage begins to really suffer if it doesn't get strong sun. So much so, that it sometimes would be a slow death sentence if I repotted them in Autumn with the weaker and lower levels of winter sun that ensued.