Will those maples grow well in the ground - over a slab?
I've got a Live Oak down in a bed, over a slab. The slab definitely facilitates a radial-shaped root spread. And, the thing is putting on wood a LOT faster than if I had kept it in a grow-out pot.
*If* I ever win the lotto, I'm gonna build a specific use raised masonry bed with most of its hight filled in with crushed stone, a layer of slate and space for 6-8 inches of soil for freshly coppiced trees.
As Auballagh notes there is an interim space bigger than any pot is going to provide, but shallower than bed planting that could well move an outdoors tree along better-faster than either pot or bed does now.
Do I know this? No. But I'm willing to side with his premis.