Our build or UE 3 does not support Mac clients and it is a non trivial thing to change the engine build to one that does. It is something we are interested in, but currently don't see the demand for.
As far as upgrading our engine, it remains something we continue to evaluate, but it would roughly mean a downtime of 3 to 6 months with no bug fixes, no content additions or anything of the sort as we got the game online on a new version of the engine and fixed it. They engine has changed massively over the years and upgrading will break many parts of it that will need to be hunted down and fixed. Month to month changes are usually fairly trivial with the engine, but they add up and entire features are ripped out and replaced. For example the UI system, foliage system, terrain system and more that we use are now obsolete in the latest builds of the engine. We would either need to bring the old systems forward, or start from scratch with the new systems (both of these options are not "easy"). As I said, non trivial. But it remains something we continue to evaluate as we move forward.
Pretty, what do we blow up first? - Myn Donos