This page says up to 60% in a combined cycle (I think that means adding a secondary steam turbine?). It also says a standard one can produce 35-40% thermal efficiency. Though on other pages I find efficiencies stated anywhere from 15%-85%...
Gas turbine - Wikipedia, the free encyclopedia
This page says even over 60% is possible:
Turbine Efficiency
Also interesting:
Turbine Calculation
I guess the big question is, what kind of efficiency can they achieve in a microturbine that fits and works in a car?