I really like the idea of a LNC4P with a Williams prop jet engine.  I'll bet
it gets much better specific fuel consumption than the Walters (sp?).
Admittedly, I know very little about either engine but I have lots of time
behind PT-6's, like 1400 hrs. so I do understand turbines somewhat.
George correctly states that temperature limits are what holds back turbine
technology but there are other ways to improve SFC.  For instance in large
jet engines I have heard it stated that a .001" improvement in the turbine
wheels' end gap can be worth a 1% improvement.
Compare the Allison C-30 to an equally rated PT-6 and I'll bet its SFC is
much better.  The engine's basic design can have a great effect as well as
its time in service and the condition of the wheels.  Bypass ratio, too, has
a terrific effect on SFC.  The old Turbomecca Marbor VI-C in the Paris II
was a gross fuel hog with its early technology.  It was a no bypass engine
with a low engine pressure ratio and a large fuel hose.  That's how I
picture the Walters in my minds eye.  A PT-6 clone with much worse design
characteristics that is well worn to begin with.
My guess is that the Williams turboprop will have a 30% better range
capability than the Walters.
If so that would be a start on the problem.  More fuel capacity will be the
clincher.  Unfortunately, that may mean a new wing.
And why stick with a free turbine for the LNC4?  Has anyone looked into
stuffing the Allison into a Lancair?  Unlike the Williams with its sticker
shock older and less expensive Allison's may be available.
Walter (would love to build a turbine next) Dodson