Static physics bodies are much faster than dynamic ones. They just provide collision and require no calculations for movement. Dynamic bodies require both.
I'd just try and be conservative. I don't change an object from a physics body into a regular object unless someone is holding it. Switching back and forth is fast but it wouldn't be so fast if you were doing it all the time. Something else to consider - when you're using a physics engine you have to let it keep control of an object unless you assume positive control of it, otherwise it won't look right. An example of this would be when I tried to cull physics bodies. If an object was off screen I'd force it to sleep. It's faster than deleting the body and saves resources. The engine will already make bodies sleep if they are under a certain velocity, but I wanted to squeeze out more performance. The problem was objects off screen could be in any state - I had lots of debris that would fly off screen and get stuck until you walked up to them. It didn't look good. It presents all kinds of problems. I'd say experiment with it, you might find a solution that works for you.
I use dynamic boxes for everything. Boxes are extremely fast, way faster than trying to use convex shapes for objects. Things look just fine as boxes, they fall and bounce and roll convincingly. I included sphere support as well but I rarely use it.
Come see the WIP!