I guess nobody cares about efficiency in their model car engine, so it doesn't matter if you need to refuel every 5-10 minutes. But that would be a problem for pretty much any other use case.
Does anyone know how the efficiency per liter of engine volume compares to these small turbine engines?
Hard to compare as it depends on application, the shaft speed differential is huge or you're comparing jet thrust to propeller thrust. I don't think small turbines (like RC jet turbine size) are usually very efficient as they are working against the surface area ratio of heat loss through the chamber walls and Reynolds number effects on the turbine blades.
How long can these engines be ran at rated power before you have to overhaul or replace?
30-50hr before they need to be re-sleeved and given a new piston