The Glass shader behaves the same on imported vs. native objects, however *some* native objects are Displaceable, and these are dynamically subdividable when with Ray Trace Objects enabled, whereas imported objects are not (you have to disable Ray Trace Objects either globally or on a per-object basis), and the results can be messy/ugly. It sounds like what you are seeing is the difference between a native displaceable object and a standard imported object.
- Oshyan