Is there much difference in growth rates growing plants in the ground compared to growing in a large pot and allowing the roots to grow from the pot and into the ground below?
It Depends!
If the purpose of retaining the pot is to help form the root ball with feeder roots, then the roots extending into the ground will be cut on a regular basis to promote feeder roots. For this reason the growth may be slower.
If retaining the pot means the tree receives more care on a regular basis with lots of water and fertiliser than the growth may be greater than if just planted in the ground and left to its own devices.
Given equal care, the inground growth will be faster! In Bonsai rarely is fast growth the only objective. In fact, it can often complicate things and actually slow progress towards development and refinement!
The key is proper management of the growth pattern.
For example. If the trunk grows fast at the expense of compact root ball and or lower branching and interior foliage. In this case one spends more time later developing the compact root ball, and or the lower branching and interior foliage. So one really needs to ask the question if the combined development time is shorter with in ground growing or some other combination.
I prefer a combination that develops a basic root ball, trunk movement and lower branching before focusing on strictly trunk size with in ground or container in the ground growing. This is partly for convenience as i detest laying on the ground to prune developing trees.