Since I only had access to kMeshData in the deformer, I couldn't create a dagPath (will talk a bit more about that later) but in terms of writing a utility script or testing the examples in the script editor, these will work just fine.
Method 1 - Using the boundingBox method of a MFnDagNode function set
The bounding box of a transform relates to its children, and the bounding box of the mesh relates to its components. Since a mesh needs a transform in order to live in the scene, it can be confusing which one relates to what exactly but all we need to really know is that the values displayed in the attribute editor are in object space.
Since the mesh is a child of the transform, the bounding box co-ordinates represented on the transform will inform us of the bounding box of the mesh.
However, as I mentioned above, this will be in object space. So if the transform is parented to another transform and the object is arbitrarily positioned in the world, these values may not be of much use depending on the circumstance.
Group the polyCube, then move that group somewhere in the world. If you check the min/max values you will notice they don't change.
Method 2 - Building a bounding box from points
If we again group the polyCube and move the group somewhere in the world, this time we will have the world space co-ordinates for the min and max.
Method 3 - Creating world-space bounding box using min/max point matrix multiplication
The important part is on line 5 and line 6 where we multiply the object-space bounding box min and max points by it's world matrix (inclusive) which transforms the points to world-space.
Method 4 - transformUsing method of a MBoundingBox object
We can reduce the amount of steps further by making use of the transformUsing method. It takes a MMatrix and transforms the points for us.
Bonus - Testing intersection
We can use the intersects method to check for collision with another bounding box.
To make sure the intersect results are correct, you would need to make sure you build a world space bounding box object as well..
I'll leave that up to you :)