Thanks for the heads-up - alloys have improved again. Still, according to
Gas turbine - Wikipedia, the free encyclopedia a simple stationary turbine suitable for generators has only managed 46%. On aircraft, they are nice because they are light, reliable, and suitable for use where mach numbers matter. To get to 60% combined efficiency, the waste heat is used for a conventional steam turbine. Presumably, the Napier Nomad could have still driven a steam or stirling engine if it was on the ground - I'll be watching for news of secondary generation from diesels, and listening for whines where I have come to expect thumpers in remote communities.