In your code, use the static flag
BoundingSphere.useExactBounds=true;
for all bounding sphere updates to calculate the smallest posible bounding sphere for the set of points they are given.
The default for the flag is false, which makes it calculate a quick but large bounding sphere for models. This is no different than the current implimentation. Setting the flag to true will calculate the smallest sphere posible.
If your code is updating model bounds every frame for a sphere, you'll want to use the quick (BoundingSphere.useExactBounds=false) method. But if you're using BoundingSphere for view culling or intersection testing, you'll want to calculate the smallest sphere posible. Try it out and let me know of any bugs you find.