If I remember correctly Lance posted an explanation quite a few months back
of the why and why nots of using a variety of turbine engine.

He noted in particular that the biggest problem with using the Williams
turboprop in a Lancair IV is that the engine is too light. The completed
aircraft would'nt balance. The other issue is that I believe Williams has
made it quite clear that they don't want there engines in experimental
aircraft.

Ted Stanley
Direct Flight, Inc.