But I somehow doubt that TSFC increases freely as the engine is increased. Say you were to install a GE90-115 on really small plane... Cessna Citation would be definitely too small, maybe Dassault Falcon.
When GE90-115 is running on idle, the thrust is very small. The fuel burn would be less than in normal cruise or takeoff, but I think still appreciable - and the TSFC would be high on idle engines. Right?